docs: Fix a missing language in a code-block

This should fix the docs build.
Spotted by spstarr, thanks!

llvm-svn: 264209
diff --git a/llvm/docs/ProgrammersManual.rst b/llvm/docs/ProgrammersManual.rst
index 668a0a2..88a8f88 100644
--- a/llvm/docs/ProgrammersManual.rst
+++ b/llvm/docs/ProgrammersManual.rst
@@ -359,7 +359,7 @@
 Error values can be implicitly converted to bool: true for error, false for
 success, enabling the following idiom:
 
-.. code-block::
+.. code-block:: c++
 
   if (auto Err = mayFail())
     return Err;