gh-37166: add Pari file to L-function documentation
    
just adding one missing link in the doc

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: #37166
Reported by: Frédéric Chapoton
Reviewer(s): Alex J Best, AurelPage

src/doc/en/reference/lfunctions/index.rst

@@ -11,5 +11,6 @@ with `L`-functions.
    sage/lfunctions/sympow
    sage/lfunctions/dokchitser
    sage/lfunctions/zero_sums
+   sage/lfunctions/pari
 
 .. include:: ../footer.txt