Improve wording and "fix example" (remove 3.13) on testing against pre-release (#979)

* Improve wording and "fix example" (remove 3.13) on testing against pre-releases

3.13 was added in a mass tune up in 0b93645 , without
adjusting the wording. With 3.13 listed there too, example does not really make much
sense. So I decided to make it explicit in wording and remove 3.13, so whenever next
refactoring to add 3.14 to be added to every line where 3.13 is -- this would not
even come to attention

* Update to use 3.14 not 3.12 as an example for pre-release

Author: yarikoptic

docs/advanced-usage.md

@@ -632,8 +632,8 @@ If the runner is not able to access github.com, any Python versions requested du
 The `allow-prereleases` flag defaults to `false`.
 If `allow-prereleases` is set to `true`, the action will allow falling back to pre-release versions of Python when a matching GA version of Python is not available.
 This allows for example to simplify reuse of `python-version` as an input of nox for pre-releases of Python by not requiring manipulation of the `3.y-dev` specifier.
-For CPython, `allow-prereleases` will only have effect for `x.y` version range (e.g. `3.12`).
-Let's say that python 3.12 is not generally available, the following workflow will fallback to the most recent pre-release of python 3.12:
+For CPython, `allow-prereleases` will only have effect for `x.y` version range (e.g. `3.14`).
+Let's say that in the past, when python 3.14 was not yet generally available, the following workflow would have fallback to the most recent pre-release of python 3.14:
 ```yaml
 jobs:
   test:
@@ -643,7 +643,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [Ubuntu, Windows, macOS]
-        python_version: ["3.11", "3.12", "3.13"]
+        python_version: ["3.14"]
 
     steps:
       - uses: actions/checkout@v5