dsolve: doc: improve example code quoting
dsolve: doc: improve example code quoting

Improve the quoting of example codes in the linear/nonlinear doc
fix 65231c57e8289bcd19d7c0cc482500d2b67f8bd6.

wyom/sympy,emon10005/sympy,jbbskinny/sympy,saurabhjn76/sympy,mafiya69/sympy,kevalds51/sympy,rahuldan/sympy,ga7g08/sympy,kaushik94/sympy,wanglongqi/sympy,yukoba/sympy,postvakje/sympy,sahmed95/sympy,asm666/sympy,grevutiu-gabriel/sympy,debugger22/sympy,Davidjohnwilson/sympy,jaimahajan1997/sympy,hargup/sympy,Arafatk/sympy,atreyv/sympy,emon10005/sympy,Davidjohnwilson/sympy,Titan-C/sympy,sampadsaha5/sympy,grevutiu-gabriel/sympy,sahmed95/sympy,MechCoder/sympy,Shaswat27/sympy,atsao72/sympy,madan96/sympy,kevalds51/sympy,aktech/sympy,emon10005/sympy,Vishluck/sympy,VaibhavAgarwalVA/sympy,Gadal/sympy,farhaanbukhsh/sympy,VaibhavAgarwalVA/sympy,mafiya69/sympy,toolforger/sympy,sahmed95/sympy,garvitr/sympy,hargup/sympy,lindsayad/sympy,mafiya69/sympy,bukzor/sympy,MechCoder/sympy,yashsharan/sympy,wyom/sympy,Davidjohnwilson/sympy,AkademieOlympia/sympy,abhiii5459/sympy,souravsingh/sympy,rahuldan/sympy,skidzo/sympy,skidzo/sympy,Designist/sympy,ga7g08/sympy,AkademieOlympia/sympy,AkademieOlympia/sympy,Gadal/sympy,kumarkrishna/sympy,pandeyadarsh/sympy,Curious72/sympy,hargup/sympy,Titan-C/sympy,jbbskinny/sympy,postvakje/sympy,pandeyadarsh/sympy,shikil/sympy,MechCoder/sympy,Gadal/sympy,chaffra/sympy,ahhda/sympy,kumarkrishna/sympy,maniteja123/sympy,abloomston/sympy,Titan-C/sympy,moble/sympy,grevutiu-gabriel/sympy,Curious72/sympy,chaffra/sympy,sahilshekhawat/sympy,yashsharan/sympy,kaichogami/sympy,ahhda/sympy,chaffra/sympy,bukzor/sympy,lindsayad/sympy,Arafatk/sympy,ChristinaZografou/sympy,pandeyadarsh/sympy,cswiercz/sympy,aktech/sympy,abhiii5459/sympy,toolforger/sympy,oliverlee/sympy,abloomston/sympy,sahilshekhawat/sympy,drufat/sympy,atreyv/sympy,asm666/sympy,Vishluck/sympy,wyom/sympy,bukzor/sympy,kaichogami/sympy,mcdaniel67/sympy,ga7g08/sympy,drufat/sympy,farhaanbukhsh/sympy,yukoba/sympy,kumarkrishna/sympy,garvitr/sympy,cswiercz/sympy,debugger22/sympy,souravsingh/sympy,atsao72/sympy,atreyv/sympy,sahilshekhawat/sympy,sampadsaha5/sympy,yukoba/sympy,jerli/sympy,garvitr/sympy,ChristinaZografou/sympy,shikil/sympy,iamutkarshtiwari/sympy,iamutkarshtiwari/sympy,skidzo/sympy,moble/sympy,oliverlee/sympy,jbbskinny/sympy,mcdaniel67/sympy,saurabhjn76/sympy,wanglongqi/sympy,abhiii5459/sympy,toolforger/sympy,mcdaniel67/sympy,kaichogami/sympy,maniteja123/sympy,aktech/sympy,lindsayad/sympy,Designist/sympy,sampadsaha5/sympy,souravsingh/sympy,iamutkarshtiwari/sympy,VaibhavAgarwalVA/sympy,oliverlee/sympy,postvakje/sympy,jerli/sympy,madan96/sympy,jerli/sympy,ahhda/sympy,asm666/sympy,Shaswat27/sympy,Vishluck/sympy,kaushik94/sympy,Curious72/sympy,yashsharan/sympy,Designist/sympy,cswiercz/sympy,saurabhjn76/sympy,Shaswat27/sympy,rahuldan/sympy,kevalds51/sympy,ChristinaZografou/sympy,madan96/sympy,jaimahajan1997/sympy,Arafatk/sympy,wanglongqi/sympy,moble/sympy,kaushik94/sympy,debugger22/sympy,maniteja123/sympy,abloomston/sympy,farhaanbukhsh/sympy,jaimahajan1997/sympy,drufat/sympy,atsao72/sympy,shikil/sympy