Add missing ".adoc" extension in handler_page.go
Add missing ".adoc" extension in handler_page.go

Commit 358dcce supposedly added ".adoc" extension recognition
for AsciiDoc, but one place was missed.

Thanks to @sjfloat for reporting the bug!
See discussions at #470.

mitchchn/hugo,fedelibre/hugo,fhergomez/hugo,tubo028/hugo,badoet/hugo,Zenithar/hugo,fedelibre/hugo,sprig/hugo,kujohn/hugo,gohugoio/hugo,soundrussian/hugo,fedelibre/hugo,abourget/hugo,fhergomez/hugo,tatsushid/hugo,vbatoufflet/hugo,halostatue/hugo,tjyang/hugo,MarkDBlackwell/hugo,sprig/hugo,abourget/hugo,derekperkins/hugo,coderzh/hugo,anthonyfok/hugo,gohugoio/hugo,kujohn/hugo,anthonyfok/hugo,anthonyfok/hugo,mdhender/hugo,ifraixedes/hugo,digitalcraftsman/hugo,badoet/hugo,vbatoufflet/hugo,mitchchn/hugo,tubo028/hugo,ryanclarke/hugo,ifraixedes/hugo,Zenithar/hugo,ffrizzo/hugo,coderzh/hugo,jygastaud/hugo,tatsushid/hugo,derekperkins/hugo,jygastaud/hugo,bogem/hugo,mdhender/hugo,tischda/hugo,bramp/hugo,ffrizzo/hugo,melo/hugo,gohugoio/hugo,digitalcraftsman/hugo,tjyang/hugo,mayah/hugo,mayah/hugo,bramp/hugo,melo/hugo,halostatue/hugo,mdhender/hugo,soundrussian/hugo,fedelibre/hugo,MarkDBlackwell/hugo,bogem/hugo,mdhender/hugo,tischda/hugo,ryanclarke/hugo