Edit the Vercel preview workflow

Due to gravitational/docs#504, we can label the
default docs version something other than `[0-9]+.x`. As a result, the
`preview` submodule maps to the docs root instead of `/ver/preview`.
This change edits the message posted by the Vercel preview workflow to
accommodate this change.

.github/workflows/vercel-preview.yaml

@@ -61,5 +61,5 @@ jobs:
               issue_number: context.issue.number,
               owner: context.repo.owner,
               repo: context.repo.repo,
-              body: `🤖 Vercel preview here: ${previewUrl}/docs/ver/preview`
+              body: `🤖 Vercel preview here: ${previewUrl}/docs`
             })