Update docs url in the README for signature (#2352)

Author: r0b1n

packages/pluggableWidgets/signature-web/README.md

@@ -1,3 +1 @@
-<!-- TODO: Update marketplace URL -->
-
-Please see [Signature](https://docs.mendix.com/appstore/widgets/) in the Mendix documentation for details.
+Please see [Signature](https://docs.mendix.com/appstore/widgets/signature) in the Mendix documentation for details.