.github: correct toxenv entry for 3.13

While adding a job to run Python v3.13 container, the target tox
environment was not updated to run v3.13; correcting.

Signed-off-by: James Knight <james.d.knight@live.com>

.github/workflows/build.yml

@@ -40,7 +40,7 @@ jobs:
             - { os:  ubuntu-latest, img: "python:3.10-slim",         python: "3.10", toxenv:  py310, cache: ~/.cache/pip }
             - { os:  ubuntu-latest, img: "python:3.11-slim",         python: "3.11", toxenv:  py311, cache: ~/.cache/pip }
             - { os:  ubuntu-latest, img: "python:3.12-slim",         python: "3.12", toxenv:  py312, cache: ~/.cache/pip }
-            - { os:  ubuntu-latest, img: "python:3.13-slim",         python: "3.13", toxenv:  py312, cache: ~/.cache/pip }
+            - { os:  ubuntu-latest, img: "python:3.13-slim",         python: "3.13", toxenv:  py313, cache: ~/.cache/pip }
             - { os:   macos-latest,                                  python: "3.12", toxenv:  py312, cache: ~/Library/Caches/pip }
             - { os: windows-latest,                                  python: "3.12", toxenv:  py312, cache: ~\AppData\Local\pip\Cache }
             - { os:  ubuntu-latest, img: "python:3.13-slim",         toxenv:   lint, cache: ~/.cache/pip }