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

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- [x] The title is concise, informative, and self-explanatory.
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URL: sagemath#37166
Reported by: Frédéric Chapoton
Reviewer(s): Alex J Best

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