small clippy fix (#524)

diff --git a/src/functions.rs b/src/functions.rs
index be90360..b8c8255 100644
--- a/src/functions.rs
+++ b/src/functions.rs
@@ -140,7 +140,7 @@
         ctx.and_then(|ctx| {
             ctx.ctx
                 .udaf(name)
-                .map(|fun| datafusion_expr::WindowFunction::AggregateUDF(fun))
+                .map(datafusion_expr::WindowFunction::AggregateUDF)
                 .ok()
         })
     });