doc/core: Remove non-sensical heading
doc/core: Remove non-sensical heading

There was a heading underline that doesn't really make much sense in
ReST, and Sphinx was complaining about it. Just remove it to fix, which
is in line with how the ".. note::" directive is used elsewhere.

Shaswat27/sympy,grevutiu-gabriel/sympy,amitjamadagni/sympy,liangjiaxing/sympy,hrashk/sympy,MechCoder/sympy,garvitr/sympy,mafiya69/sympy,VaibhavAgarwalVA/sympy,abhiii5459/sympy,jamesblunt/sympy,MridulS/sympy,souravsingh/sympy,kumarkrishna/sympy,maniteja123/sympy,jamesblunt/sympy,sahmed95/sympy,lindsayad/sympy,grevutiu-gabriel/sympy,jbbskinny/sympy,Davidjohnwilson/sympy,sampadsaha5/sympy,yukoba/sympy,Davidjohnwilson/sympy,hrashk/sympy,wanglongqi/sympy,ga7g08/sympy,atsao72/sympy,jbbskinny/sympy,Curious72/sympy,sunny94/temp,asm666/sympy,Gadal/sympy,drufat/sympy,mafiya69/sympy,chaffra/sympy,kumarkrishna/sympy,yashsharan/sympy,drufat/sympy,iamutkarshtiwari/sympy,yashsharan/sympy,ahhda/sympy,ga7g08/sympy,Arafatk/sympy,mcdaniel67/sympy,Titan-C/sympy,Designist/sympy,kaushik94/sympy,dqnykamp/sympy,MridulS/sympy,moble/sympy,jbbskinny/sympy,postvakje/sympy,emon10005/sympy,farhaanbukhsh/sympy,AkademieOlympia/sympy,vipulroxx/sympy,MechCoder/sympy,aktech/sympy,hargup/sympy,rahuldan/sympy,Gadal/sympy,oliverlee/sympy,shipci/sympy,cswiercz/sympy,flacjacket/sympy,kaushik94/sympy,Sumith1896/sympy,debugger22/sympy,hrashk/sympy,farhaanbukhsh/sympy,saurabhjn76/sympy,yashsharan/sympy,moble/sympy,vipulroxx/sympy,maniteja123/sympy,saurabhjn76/sympy,ChristinaZografou/sympy,garvitr/sympy,drufat/sympy,wyom/sympy,souravsingh/sympy,VaibhavAgarwalVA/sympy,shikil/sympy,cccfran/sympy,moble/sympy,iamutkarshtiwari/sympy,bukzor/sympy,Mitchkoens/sympy,toolforger/sympy,kaichogami/sympy,pandeyadarsh/sympy,yukoba/sympy,AkademieOlympia/sympy,aktech/sympy,atreyv/sympy,VaibhavAgarwalVA/sympy,skidzo/sympy,cswiercz/sympy,bukzor/sympy,amitjamadagni/sympy,Arafatk/sympy,abloomston/sympy,skidzo/sympy,lindsayad/sympy,jerli/sympy,kevalds51/sympy,jaimahajan1997/sympy,pandeyadarsh/sympy,meghana1995/sympy,vipulroxx/sympy,jamesblunt/sympy,bukzor/sympy,souravsingh/sympy,Curious72/sympy,postvakje/sympy,oliverlee/sympy,abhiii5459/sympy,pandeyadarsh/sympy,beni55/sympy,wanglongqi/sympy,hargup/sympy,meghana1995/sympy,skidzo/sympy,abloomston/sympy,Sumith1896/sympy,farhaanbukhsh/sympy,ChristinaZografou/sympy,shikil/sympy,rahuldan/sympy,shipci/sympy,kmacinnis/sympy,kaushik94/sympy,atsao72/sympy,kmacinnis/sympy,MechCoder/sympy,srjoglekar246/sympy,lidavidm/sympy,asm666/sympy,Titan-C/sympy,AkademieOlympia/sympy,Vishluck/sympy,AunShiLord/sympy,madan96/sympy,maniteja123/sympy,hargup/sympy,abloomston/sympy,Sumith1896/sympy,shipci/sympy,beni55/sympy,dqnykamp/sympy,sahmed95/sympy,Gadal/sympy,debugger22/sympy,sahmed95/sympy,emon10005/sympy,beni55/sympy,iamutkarshtiwari/sympy,pbrady/sympy,chaffra/sympy,jerli/sympy,cccfran/sympy,Mitchkoens/sympy,oliverlee/sympy,kumarkrishna/sympy,shikil/sympy,wyom/sympy,Curious72/sympy,meghana1995/sympy,atreyv/sympy,skirpichev/omg,asm666/sympy,atsao72/sympy,liangjiaxing/sympy,kaichogami/sympy,sunny94/temp,mcdaniel67/sympy,atreyv/sympy,Designist/sympy,toolforger/sympy,Mitchkoens/sympy,kmacinnis/sympy,Davidjohnwilson/sympy,wanglongqi/sympy,abhiii5459/sympy,rahuldan/sympy,sampadsaha5/sympy,emon10005/sympy,lidavidm/sympy,lindsayad/sympy,sunny94/temp,ahhda/sympy,sampadsaha5/sympy,Vishluck/sympy,garvitr/sympy,lidavidm/sympy,sahilshekhawat/sympy,mcdaniel67/sympy,pbrady/sympy,AunShiLord/sympy,ChristinaZografou/sympy,AunShiLord/sympy,liangjiaxing/sympy,saurabhjn76/sympy,ahhda/sympy,Arafatk/sympy,madan96/sympy,Shaswat27/sympy,wyom/sympy,sahilshekhawat/sympy,postvakje/sympy,MridulS/sympy,jaimahajan1997/sympy,mafiya69/sympy,cswiercz/sympy,yukoba/sympy,kaichogami/sympy,jerli/sympy,chaffra/sympy,madan96/sympy,cccfran/sympy,Titan-C/sympy,Shaswat27/sympy,Designist/sympy,aktech/sympy,diofant/diofant,debugger22/sympy,toolforger/sympy,dqnykamp/sympy,sahilshekhawat/sympy,jaimahajan1997/sympy,kevalds51/sympy,ga7g08/sympy,Vishluck/sympy,pbrady/sympy,grevutiu-gabriel/sympy,kevalds51/sympy