vf_unsharp: fix copy&paste error in chroma_amount param docs

Signed-off-by: Stefano Sabatini <stefano.sabatini-lala@poste.it>

doc/filters.texi

@@ -1802,7 +1802,7 @@ and 13, default value is 0.
 Set the chroma matrix vertical size. It can be an integer between 3
 and 13, default value is 0.
 
-@item luma_amount
+@item chroma_amount
 Set the chroma effect strength. It can be a float number between -2.0
 and 5.0, default value is 0.0.