Fix comment in pg_upgrade.h.

Contrary to what the comment for the "check" struct member claims,
'pg_upgrade --check' performs only the checks and does not ask the
user for permission to make changes.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/ZnHk7ci5IuTWVc_c%40nathan

src/bin/pg_upgrade/pg_upgrade.h

@@ -321,8 +321,7 @@ typedef struct
 */
 typedef struct
 {
-	bool		check;			/* true -> ask user for permission to make
-								 * changes */
+	bool		check;			/* check clusters only, don't change any data */
 	bool		do_sync;		/* flush changes to disk */
 	transferMode transfer_mode; /* copy files or link them? */
 	int			jobs;			/* number of processes/threads to use */