bpo-44030: Fix formatting error in exceptions docs (GH-25929)

Author: miguendes

Doc/library/exceptions.rst

@@ -96,7 +96,7 @@ The following exceptions are used mostly as base classes for other exceptions.
       instance of ``OtherException`` while preserving the traceback.  Once
       raised, the current frame is pushed onto the traceback of the
       ``OtherException``, as would have happened to the traceback of the
-      original ``SomeException`` had we allowed it to propagate to the caller.
+      original ``SomeException`` had we allowed it to propagate to the caller. ::
 
          try:
              ...