Separate code-block tag with a newline to fix code snippet html output

clang/include/clang/Basic/AttrDocs.td

@@ -1084,6 +1084,7 @@ not made control-dependent on any additional values, e.g., unrolling a loop
 executed by all work items.
 
 Sample usage:
+
 .. code-block:: c
 
   void convfunc(void) __attribute__((convergent));