functions_aggregate::expr_fn::approx_distinct should expose the function, not the module

[Michael-J-Ward]

Describe the bug

Currently, functions_aggregate::expr_fn::approx_distinct points to the module, not the fluent API function.

datafusion/datafusion/functions-aggregate/src/lib.rs

Lines 92 to 96 in 20b298e

	pub mod expr_fn {
	pub use super::approx_distinct;
	pub use super::approx_median::approx_median;
	pub use super::approx_percentile_cont::approx_percentile_cont;
	pub use super::approx_percentile_cont_with_weight::approx_percentile_cont_with_weight;

To Reproduce

No response

Expected behavior

functions_aggregate::expr_fn::approx_distinct should reference the fluent API for creating a UDAf.

Additional context

I'll have a PR shortly.