fast-export: fix argument name in error messages

The --signed-tags argument is plural, while error messages referred
to --signed-tag (singular).  Tweak error messages to correspond to the
argument.

Signed-off-by: Paul Price <price@astro.princeton.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fast-export.c

diff --git a/builtin/fast-export.c b/builtin/fast-export.c
index 12220ad..06a5279 100644
--- a/builtin/fast-export.c
+++ b/builtin/fast-export.c
@@ -43,7 +43,7 @@
 	else if (!strcmp(arg, "strip"))
 		signed_tag_mode = STRIP;
 	else
-		return error("Unknown signed-tag mode: %s", arg);
+		return error("Unknown signed-tags mode: %s", arg);
 	return 0;
 }
 
@@ -416,7 +416,7 @@
 			switch(signed_tag_mode) {
 			case ABORT:
 				die ("Encountered signed tag %s; use "
-				     "--signed-tag=<mode> to handle it.",
+				     "--signed-tags=<mode> to handle it.",
 				     sha1_to_hex(tag->object.sha1));
 			case WARN:
 				warning ("Exporting signed tag %s",