fix: tiny typo in AggregatedStats

[haruband]

Description

This PR fix typo in AggregatedStats.

Related Issue(s)

• closes Can't get stats's minValues in add actions #1515

Documentation

[wjones127]

Thanks for doing this; this seems obvious right but I'm concerned about the fact that our unit tests didn't catch this. We are testing min values here:

delta-rs/rust/src/writer/stats.rs

Lines 662 to 664 in 549a74a

	// assert on min values
	for (k, v) in stats.min_values.iter() {
	match (k.as_str(), v) {

Do you know why this missed this? Is this a different code path?

[haruband]

@wjones127
Our unit tests have only one row group, so we couldn't test the cases which use add_assign for AggregatedStats. I added new commit to fix it.