ci/docs: fix build ref

Fix typo `inputs.ref` → `matrix.ref`.

This caused an empty string to be assigned to the checkout action's
`ref` input, meaning it used its default value `github.ref`.

Therefore all "versions" of the docs were actually building `main` 😂

Instead, we actually want to checkout the ref from the job's `matrix`.

(cherry picked from commit 95957f306b)

.github/workflows/website.yml

@@ -48,7 +48,7 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
         with:
-          ref: ${{ inputs.ref }}
+          ref: ${{ matrix.ref }}
 
         # Uses the build-docs action from the checked-out nixvim branch
       - name: Build docs