docs: update ref to charmcraft.yaml reference (canonical#1580)

This accounts for an incoming change in Charmcraft:
canonical/charmcraft#2169. The build will be
breaking until that PR is merged.

@dwilding I didn't find anything in the contribution guidelines about
branching. Let me know if it's OK to PR into main.

docs/tutorial/write-your-first-machine-charm.md

@@ -288,7 +288,7 @@ config:
 ```
 
 
-> See more: {external+charmcraft:ref}`Charmcraft | File charmcraft.yaml | Key config <recipe-key-config>`
+> See more: {external+charmcraft:ref}`Charmcraft | File charmcraft.yaml | Key config <charmcraft-yaml-key-config>`
 
 Then, in the `src/charm.py` file, update the `_on_install` function to make use of the new configuration option, as below: