Rename the CI `cargo fmt` job to `formatting`. (#728)

This brings the CI in sync with the latest Linebender standard, see e.g.
[xilem](https://github.com/linebender/xilem/blob/c3a6bd55163d1c37251bd8af47f0067be81319e5/.github/workflows/ci.yml#L73).
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ab39c0b..9b72809 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -57,7 +57,7 @@
 
 jobs:
   fmt:
-    name: cargo fmt
+    name: formatting
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4