iproute2: Fix grammar in duplicate argument error message

Change "is a garbage" to "is garbage". Because garbage is a collective
noun, it does not need the indefinite article.

Signed-off-by: Neil Svedberg <neil.svedberg@gmail.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

lib/utils.c

@@ -812,7 +812,7 @@ void duparg(const char *key, const char *arg)
 void duparg2(const char *key, const char *arg)
 {
 	fprintf(stderr,
-		"Error: either \"%s\" is duplicate, or \"%s\" is a garbage.\n",
+		"Error: either \"%s\" is duplicate, or \"%s\" is garbage.\n",
 		key, arg);
 	exit(-1);
 }