[Docs] Highlight keymap name entry (qmk#21138)

docs/newbs_building_firmware_workflow.md

@@ -85,6 +85,8 @@ Visit the [QMK Configurator](https://config.qmk.fm/#/) to create a keymap file:
 3. Customise the key layout according to your preference.
 4. Select download next to **KEYMAP.JSON** and save the JSON file into the `~/qmk_keymap/` folder.
 
+!> **Important:** Make sure that the GitHub username you use in step 2 is correct. If it is not, the build process will fail to locate your files in the right folder.
+
 ### Add a GitHub Action workflow
 
 Open the file `~/qmk_keymap/.github/workflows/build.yml` with your favorite [text editor](newbs_learn_more_resources.md#text-editor-resources), paste the following workflow content, and save it: