[FileCheck] Fix sphinx error: Make input be gas block

Summary:
Change example of input text from being llvm block to being gas block
since that text is made-up assembly.

Reviewers: jhenderson, jdenny, probinson, arichardson

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61893

llvm-svn: 360781