Use devbox run -- ... instead of devbox shell -- ... (#1352)

## Summary

Docs updated for the GitHub Action.

```diff
- run: devbox shell -- echo "done!"
+ run: devbox run -- echo "done!"
```

## How was it tested?

I ran the command locally.

Signed-off-by: Mike Nikles <[email protected]>

Author: mikenikles

docs/app/docs/continuous_integration/github_action.md

@@ -36,7 +36,7 @@ jobs:
         uses: jetpack-io/[email protected]
 
       - name: Run arbitrary commands
-        run: devbox shell -- echo "done!"
+        run: devbox run -- echo "done!"
 
       - name: Run a script called test
         run: devbox run test
@@ -61,4 +61,4 @@ An example of this configuration is below:
     project-path: 'path-to-folder'
     enable-cache: true
     devbox-version: '0.2.2'
-```
+```