Fix doc Set#each returns nil instead of self

[kachick]

I think current spec is correct.

crystal/spec/std/set_spec.cr

Lines 328 to 331 in 9d7e162

	set.each do |v|
	v.should eq(i)
	i += 1
	end.should be_nil

My understanding is below. If it is not correct, I'll fix the implementation and spec instead of documentation changes.

Returning self on `#each` method is a Ruby conventional interface.
Crystal style is returning nil on `#each` method.

crystal/src/hash.cr

Lines 1259 to 1263 in 9d7e162

	def each : Nil
	each_entry_with_index do |entry, i|
	yield({entry.key, entry.value})
	end
	end

[sdogruyol]

Thank you @kachick 🙏

[bcardiff]

Leftover since #3826 😅

[kachick]

Thank you all!

@bcardiff Thank you for letting me know 🙏