Small errors in the documentation for `rchunks_exact` and `rchunks_exact_mut`

[cyrozap]

While reading through some documentation I found what appear to be a few copy/paste errors.

rchunks_exact

"beginning of the slice" should be "end of the slice" here:

rust/src/libcore/slice/mod.rs

Lines 840 to 841 in 9387927

	/// Returns an iterator over `chunk_size` elements of the slice at a time, starting at the
	/// beginning of the slice.

The second " of the slice" should be removed from the end of this sentence:

rust/src/libcore/slice/mod.rs

Lines 851 to 852 in 9387927

	/// chunk, and [`chunks_exact`] for the same iterator but starting at the beginning of the
	/// slice of the slice.

rchunks_exact_mut

The second " of the slice" should be removed from the end of this sentence:

rust/src/libcore/slice/mod.rs

Lines 892 to 893 in 9387927

	/// smaller chunk, and [`chunks_exact_mut`] for the same iterator but starting at the beginning
	/// of the slice of the slice.

[hellow554]

Hey @cyrozap! You already figured out the correct locations in the sourcecode, why don't you open a PR and fix that? It would be a easy first PR for you and a great opportunity to contribute to Rust.
Is that something you can do?