fix link to vendor docs in README

Updated the link to the vendor docs to point towards the vendor directory in the Tekton Chains repository, previously this would redirect to https://github.com/docs/vendor which led to a 404 error.

Signed-off-by: Cameron Guest <[email protected]>

README.md

@@ -79,7 +79,7 @@ To finish setting up Chains, please complete the following steps:
 ### Vendor specific documentation
 
 Any additional documentation specific to particular cloud vendors can be found
-at [docs/vendor](/docs/vendor/).
+at [docs/vendor](https://github.com/tektoncd/chains/tree/main/docs/vendor).
 
 ## Tutorials