[Polly][docs] Fix Sphinx warning.

ReStructured Text is not Markdown.

Author: Meinersbur

polly/docs/ReleaseNotes.rst

@@ -13,9 +13,10 @@ In Polly 14 the following important changes have been incorporated.
 - The command line option -polly-opt-fusion has been removed. What the
   flag does was frequently misunderstood and is rarely useful. However,
   the functionality is still accessible using
-```
+
+  .. code-block:: console
+
     -polly-isl-arg=--no-schedule-serialize-sccs
-```
 
 - The command line option -polly-loopfusion-greedy has been added.
   This will agressively try to fuse any loop regardless of