Fix in-toto-sign snippet on wrap up page
Fix in-toto-sign snippet on wrap up page

As pointed out by @awwad the suggested snippet did not work.
This commit fixes the snippet.

The command line tool's usage message will be fixed with
in-toto/in-toto#118

in-toto/layout-web-tool,in-toto/layout-web-tool,in-toto/layout-web-tool