builtin/log.c: make usage string consistent with doc

Replace '<since>..<until>' with '<revision range>', in accordance with
the documentation.
Signed-off-by: NRamkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/log.c

@@ -36,7 +36,7 @@ static const char *fmt_patch_subject_prefix = "PATCH";
 static const char *fmt_pretty;
 
 static const char * const builtin_log_usage[] = {
-	N_("git log [<options>] [<since>..<until>] [[--] <path>...]\n")
+	N_("git log [<options>] [<revision range>] [[--] <path>...]\n")
 	N_("   or: git show [options] <object>..."),
 	NULL
 };