advice: fix an unexpected leading space

This space was introduced, presumably unintentionally, in b3b18d1
(advice: revamp advise API, 2020-03-02)

I notice this space due to confuse diff outputs while doing some
changes to enum advice_type.

As a reference, a recent change we have to that enum is:

    $ git show 35f0383

    ...
    diff --git a/advice.h b/advice.h
    index 0f58416..2affbe1 100644
    --- a/advice.h
    +++ b/advice.h
    @@ -49,6 +49,7 @@ struct string_list;
	    ADVICE_UPDATE_SPARSE_PATH,
	    ADVICE_WAITING_FOR_EDITOR,
	    ADVICE_SKIPPED_CHERRY_PICKS,
    +       ADVICE_WORKTREE_ADD_ORPHAN,
     };

Note the hunk header, instead of a much more expected:

    @@ -49,6 +49,7 @@ enum advice_type

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rjusto

advice.h

@@ -10,7 +10,7 @@ struct string_list;
  * Add the new config variable to Documentation/config/advice.txt.
  * Call advise_if_enabled to print your advice.
  */
- enum advice_type {
+enum advice_type {
 	ADVICE_ADD_EMBEDDED_REPO,
 	ADVICE_ADD_EMPTY_PATHSPEC,
 	ADVICE_ADD_IGNORED_FILE,