builtin-merge: fix a typo in an error message

Signed-off-by: NAllan Caffee <allan.caffee@gmail.com>
Acked-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-merge.c

@@ -703,7 +703,7 @@ static int suggest_conflicts(void)
 
 	fp = fopen(git_path("MERGE_MSG"), "a");
 	if (!fp)
-		die("Could open %s for writing", git_path("MERGE_MSG"));
+		die("Could not open %s for writing", git_path("MERGE_MSG"));
 	fprintf(fp, "\nConflicts:\n");
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];