Cleanup: use single back-tick quoting in comments

While doxygen supports both, conform to our style guide.

Note that single back-tick's are already used in a majority of comments.

source/blender/python/mathutils/mathutils.c

@@ -78,7 +78,7 @@ static int mathutils_array_parse_fast(float *array,
 }
 
 /**
- * helper function that returns a Python ``__hash__``.
+ * helper function that returns a Python `__hash__`.
  *
  * \note consistent with the equivalent tuple of floats (CPython's 'tuplehash')
  */