remove trailing comma

fixes #98

Author: Matt Leathes

json/help-install.json

@@ -5,6 +5,6 @@
         "install [--dry-run, --compatible]",
         "install <plugin-name>",
         "install <plugin-name>#<semver>",
-        "install <plugin-name>@<semver>",
+        "install <plugin-name>@<semver>"
     ]
 }