Fix: Correct basic_charset operator

[alexbarev]

Should have returned result.

StringZilla/include/stringzilla/stringzilla.hpp

Lines 308 to 312 in 645539b

	basic_charset operator|(basic_charset other) const noexcept {
	basic_charset result = *this;
	result.bitset_._u64s[0] |= other.bitset_._u64s[0], result.bitset_._u64s[1] |= other.bitset_._u64s[1],
	result.bitset_._u64s[2] |= other.bitset_._u64s[2], result.bitset_._u64s[3] |= other.bitset_._u64s[3];
	return *this;

[ashvardanian]

Much appreciated, @alexbarev!