Fix documentation status badge
Fix documentation status badge

Change back badge image url to readthedocs

Fixes https://github.com/coala/coala/issues/3437

Asalle/coala,aptrishu/coala,damngamerz/coala,coala/coala,shreyans800755/coala,MariosPanag/coala,RJ722/coala,refeed/coala,meetmangukiya/coala,Balaji2198/coala,Balaji2198/coala,coala-analyzer/coala,rimacone/testing2,CruiseDevice/coala,yashtrivedi96/coala,impmihai/coala,yashtrivedi96/coala,jayvdb/coala,nemaniarjun/coala,CruiseDevice/coala,Nosferatul/coala,impmihai/coala,RJ722/coala,CruiseDevice/coala,arjunsinghy96/coala,NiklasMM/coala,shreyans800755/coala,yashLadha/coala,arush0311/coala,meetmangukiya/coala,shreyans800755/coala,kartikeys98/coala,coala-analyzer/coala,netman92/coala,Asnelchristian/coala,Shade5/coala,nemaniarjun/coala,aptrishu/coala,kartikeys98/coala,rimacone/testing2,coala-analyzer/coala,arush0311/coala,aptrishu/coala,ayushin78/coala,karansingh1559/coala,kartikeys98/coala,jayvdb/coala,sagark123/coala,coala/coala,refeed/coala,JohnS-01/coala,damngamerz/coala,karansingh1559/coala,arjunsinghy96/coala,jayvdb/coala,RJ722/coala,MariosPanag/coala,arjunsinghy96/coala,Asalle/coala,Balaji2198/coala,sagark123/coala,NiklasMM/coala,rimacone/testing2,incorrectusername/coala,incorrectusername/coala,SanketDG/coala,yashtrivedi96/coala,coala/coala,Nosferatul/coala,arush0311/coala,JohnS-01/coala,sagark123/coala,MariosPanag/coala,incorrectusername/coala,SanketDG/coala,nemaniarjun/coala,karansingh1559/coala,NiklasMM/coala,yashLadha/coala,netman92/coala,refeed/coala,Asnelchristian/coala,netman92/coala,Asnelchristian/coala,yashLadha/coala,damngamerz/coala,impmihai/coala,SanketDG/coala,Asalle/coala,ayushin78/coala,Shade5/coala,Shade5/coala,Nosferatul/coala,JohnS-01/coala,meetmangukiya/coala,ayushin78/coala