[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
diff --git a/llvm/docs/CommandGuide/FileCheck.rst b/llvm/docs/CommandGuide/FileCheck.rst
index 55c7f31..bc216be 100644
--- a/llvm/docs/CommandGuide/FileCheck.rst
+++ b/llvm/docs/CommandGuide/FileCheck.rst
@@ -593,13 +593,13 @@
 
 The above example would match the line:
 
-.. code-block:: llvm
+.. code-block:: gas
 
     add r5, r5, r6
 
 but would not match the line:
 
-.. code-block:: llvm
+.. code-block:: gas
 
     add r5, r5, r7