refs.h: fix misspelt "occurred" in a comment

Signed-off-by: Peter Colberg <peter@colberg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -345,7 +345,7 @@ struct ref_transaction *ref_transaction_begin(struct strbuf *err);
  *     msg -- a message describing the change (for the reflog).
  *
  *     err -- a strbuf for receiving a description of any error that
- *         might have occured.
+ *         might have occurred.
  *
  * The functions make internal copies of refname and msg, so the
  * caller retains ownership of these parameters.