chore: update revision in tox.ini for mkdocs-mermaid-plugin repo

Before this commit, there was no revision identifier in the installation command
for the package mkdocs-mermaid-plugin in the 'docs' and the 'docs-serve' tox
testenvs. However, it is good practice to point to a specific commit rather than
the default branch, whose content could change without notice.

This commit adds the revision to the URL pointing to the mkdocs-mermaid-plugin
GitHub repository. In particular, this commit adds 'abf1439' at the end of the
URL of the GitHub repository of the package. The URL to the commit is:

  pugong/mkdocs-mermaid-plugin@abf1439

The change is applied in both 'docs' and 'docs-serve'.

tox.ini

@@ -280,15 +280,15 @@ skipsdist = True
 skip_install = True
 deps =
     tomte[docs]==0.2.13
-commands = pip3 install git+https://github.com/pugong/mkdocs-mermaid-plugin.git#egg=mkdocs-mermaid-plugin
+commands = pip3 install git+https://github.com/pugong/mkdocs-mermaid-plugin.git@abf1439#egg=mkdocs-mermaid-plugin
            mkdocs build --clean
 
 [testenv:docs-serve]
 skipsdist = True
 skip_install = True
 deps =
     tomte[docs]==0.2.13
-commands = pip3 install git+https://github.com/pugong/mkdocs-mermaid-plugin.git#egg=mkdocs-mermaid-plugin
+commands = pip3 install git+https://github.com/pugong/mkdocs-mermaid-plugin.git@abf1439#egg=mkdocs-mermaid-plugin
            mkdocs build --clean
            python -c 'print("###### Starting local server. Press Control+C to stop server ######")'
            mkdocs serve -a localhost:8080