Use more legible syntax

Author: hynek

.github/CONTRIBUTING.md

@@ -54,7 +54,7 @@ layout python python$(cat .python-version-default)
 or, if you like [*uv*](https://github.com/astral-sh/uv):
 
 ```bash
-test -d .venv || uv venv --python python`cat .python-version-default`
+test -d .venv || uv venv --python python$(cat .python-version-default)
 . .venv/bin/activate
 ```