Enhancements for PullRequest Resource docs

It seems like users may not able able to understand
how to use the `PullRequestResource` with secret configuration.

This patch updates the docs with example Task with Secret
to demonstrate how to use the PR Resource.

Fixes tektoncd#1275

docs/resources.md

@@ -370,6 +370,15 @@ spec:
  - fieldName: githubToken
  secretName: github-secrets
  secretKey: token
+---
+apiVersion: v1
+kind: Secret
+metadata:
+ name: github-secrets
+type: Opaque
+data:
+ token: github_personal_access_token_secret # in base64 encoded form
+
 ```
 
 Params that can be added are the following:
@@ -402,7 +411,7 @@ provider statuses, see
 #### GitHub
 
 The Pull Request resource will look for GitHub OAuth authentication tokens in
-spec secrets with a field name called `authToken`.
+spec secrets with a field name called `githubToken`.
 
 URLs should be of the form: https://github.com/tektoncd/pipelines/pulls/1