Improve error message when docs need fixing with Prettier (#17907)

.github/workflows/docs.yml

@@ -20,5 +20,11 @@ jobs:
         with:
           version: 9
 
-      - run: pnpm dlx prettier . --check
+      - run: |
+          pnpm dlx prettier . --check || {
+            echo "To fix, run from the root of the zed repo:"
+            echo "  cd docs && pnpm dlx prettier . --write && cd .."
+            false
+          }
+
         working-directory: ./docs