Document -purpose option in usage string.

apps/verify.c

diff --git a/apps/verify.c b/apps/verify.c
index 267ee4e..8d211e0 100644
--- a/apps/verify.c
+++ b/apps/verify.c
@@ -186,7 +186,7 @@
 	ret=0;
 end:
 	if (ret == 1) {
-		BIO_printf(bio_err,"usage: verify [-verbose] [-CApath path] [-CAfile file] cert1 cert2 ...\n");
+		BIO_printf(bio_err,"usage: verify [-verbose] [-CApath path] [-CAfile file] [-purpose purpose] cert1 cert2 ...\n");
 		BIO_printf(bio_err,"recognized usages:\n");
 		for(i = 0; i < X509_PURPOSE_get_count(); i++) {
 			X509_PURPOSE *ptmp;