[docs] Continue working around broken Sphinx parsing of LLVM code blocks

by switching to `text` in another place.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339001 91177308-0d34-0410-b5e6-96231b3b80d8

docs/SourceLevelDebugging.rst

@@ -1506,7 +1506,7 @@ For example, here is a module before:
 
 and after running ``opt -debugify``  on it we get:
 
-.. code-block:: llvm
+.. code-block:: text
 
    define void @f(i32* %x) !dbg !6 {
    entry: