Fix incorrect conditional.
Fix incorrect conditional.

I accidentally introduced this in https://github.com/azerupi/mdBook/pull/254/commits/4525810737b678e00c23f26c218db5426df0d714.
azerupi/mdBook,azerupi/mdBook,azerupi/mdBook,azerupi/mdBook,superstring/mdBook,superstring/mdBook,superstring/mdBook,superstring/mdBook