Fix inconsistent deploy key documentation in UI

Deploy keys were added with write access to
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7383 - We still
state " Deploy keys allow read-only access to your repository." in the
UI.

This updates the deploy key UI information to reflect the docs
https://docs.gitlab.com/ce/ssh/README.html#deploy-keys

app/views/projects/deploy_keys/_index.html.haml

@@ -3,7 +3,7 @@
     %h4.prepend-top-0
       Deploy Keys
     %p
-      Deploy keys allow read-only access to your repository. Deploy keys can be used for CI, staging or production servers. You can create a deploy key or add an existing one.
+      Deploy keys allow read-only or read-write (if enabled) access to your repository. Deploy keys can be used for CI, staging or production servers. You can create a deploy key or add an existing one.
   .col-lg-9
     %h5.prepend-top-0
       Create a new deploy key for this project