[clang-tidy docs] Minor fix

llvm-svn: 279051

clang-tools-extra/docs/clang-tidy/checks/mpi-type-mismatch.rst

@@ -10,7 +10,7 @@ datatypes and null pointer constants are skipped, in the course of verification.
 
 Example:
 
-.. code:: c++
+.. code-block:: c++
 
   // In this case, the buffer type matches MPI datatype.
   char buf;