docs: remove broken image reference in contributing guide (#22013)

The "Deploy PR manually" image (`deploy-pr-manually.png`) referenced in
the contributing docs has never existed in the repository, resulting in
a broken image on the [docs
site](https://coder.com/docs/about/contributing/CONTRIBUTING#deploying-a-pr).

This PR removes the broken `<Image>` tag and ends the sentence with a
period instead. The `pr-deploy.yaml` workflow link remains intact for
users to navigate to the workflow dispatch page directly.

Created on behalf of @DavidFrawormo

Co-authored-by: blink-so[bot] <211532188+blink-so[bot]@users.noreply.github.com>

docs/about/contributing/CONTRIBUTING.md

@@ -119,9 +119,7 @@ this:
 - Run `./scripts/deploy-pr.sh`
 - Manually trigger the
   [`pr-deploy.yaml`](https://github.com/coder/coder/actions/workflows/pr-deploy.yaml)
-  GitHub Action workflow:
-
-  <Image src="./images/deploy-pr-manually.png" alt="Deploy PR manually" height="348px" align="center" />
+  GitHub Action workflow.
 
 #### Available options