docs: update document of folds (#11705)

docs/src/rust/user-guide/expressions/folds.rs

@@ -25,7 +25,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
         .lazy()
         .filter(fold_exprs(
             lit(true),
-            |acc, x| Some(acc.bitand(&x)),
+            |acc, x| acc.bitand(&x).map(Some),
             [col("*").gt(1)],
         ))
         .collect()?;