Fix mistakes in MVar documentation

[trinistr]

Fixes a couple of mistakes in documentation:

• #modify's documentation very incorrectly said that it returns modified value, when that's not the case;
• class's docs referenced #mutate instead of #modify;
• typo in a comment.

Additionally, links to README and CHANGELOG are fixed in contributing guidelines, previously they linked to non-existent files.

[eregon]

Suggested change

	Document any external behavior in the [README](/README.md).
	Document any external behavior in the [README](../README.md).

/README.md doesn't seem to work, on GitHub at least, same below of course

[trinistr]

Hmm, weird, it did work for me when I viewed CONTRIBUTING.md in the repository.

Fixed.

[eregon]

I wonder if instead the code should return the transformed value.
But looking at

concurrent-ruby/spec/concurrent/mvar_spec.rb

Lines 180 to 183 in c8f0bae

	it 'returns the unmodified value' do
	m = MVar.new(14)
	expect(m.modify { |v| v + 2 }).to eq 14
	end

it seems not, so indeed the doc change seems correct then.

[trinistr]

Actually, I noticed the problem because the documented behavior seemed weird, as getting previous value would be hard in many cases, and it did not follow the "set and get" pattern.