[Docs] Switch a code block from LLVM to text

While I can't replicate this locally, it looks like the buildbots don't
recognize the IR block in r358385 l764 as IR. Downgrade it to being just
text while I look into it.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358391 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/SourceLevelDebugging.rst b/docs/SourceLevelDebugging.rst
index 977b7b7..977c97c 100644
--- a/docs/SourceLevelDebugging.rst
+++ b/docs/SourceLevelDebugging.rst
@@ -761,7 +761,7 @@
 requries the consideraton of control flow. Consider the following IR, which
 presents several difficulties:
-.. code-block:: llvm
+.. code-block:: text
   define dso_local i32 @foo(i1 %cond, i32 %input) !dbg !12 {