Fixed _callback.md typo
Fixed _callback.md typo

Changed "Settings section" to "Application Settings" to avoid having the word section written twice.
khorne3/docs,khorne3/docs,jeffreylees/docs,sandrinodimattia/docs,Annyv2/docs,auth0/docs,sandrinodimattia/docs,Annyv2/docs,yvonnewilson/docs,yvonnewilson/docs,Catografix/docs,auth0/docs,Catografix/docs,hamzawi06/docs,miparnisari/docs,yvonnewilson/docs,hamzawi06/docs,khorne3/docs,jeffreylees/docs,hamzawi06/docs,Annyv2/docs,jeffreylees/docs,Catografix/docs,auth0/docs,sandrinodimattia/docs