Update Doc of required_indices.rs (apache#13555)

There is a mistake in the doc，missed a number

datafusion/optimizer/src/optimize_projections/required_indices.rs

@@ -33,7 +33,7 @@ use datafusion_expr::{Expr, LogicalPlan};
 ///
 /// Indices are always in order and without duplicates. For example, if these
 /// indices were added `[3, 2, 4, 3, 6, 1]`,  the instance would be represented
-/// by  `[1, 2, 3, 6]`.
+/// by  `[1, 2, 3, 4, 6]`.
 #[derive(Debug, Clone, Default)]
 pub(super) struct RequiredIndicies {
     /// The indices of the required columns in the