sign_encrypt.py: fix typo in stitch usage message

Fix typo in stitch usage message.

Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/scripts/sign_encrypt.py b/scripts/sign_encrypt.py
index 435d63b..9c37d40 100755
--- a/scripts/sign_encrypt.py
+++ b/scripts/sign_encrypt.py
@@ -245,7 +245,7 @@
     arg_add_dig(parser_digest)
 
     parser_stitch = subparsers.add_parser(
-        'stitch', aliases=['stitch-ta'], prog=parser.prog + ' stich',
+        'stitch', aliases=['stitch-ta'], prog=parser.prog + ' stitch',
         help='Generate loadable signed and encrypted TA binary image file' +
         ' from TA raw image and its signature')
     parser_stitch.set_defaults(func=command_stitch)