payments.md: remove broken links to examples
payments.md: remove broken links to examples

See https://github.com/expo/expo/commit/740517bbb9d03d03927e10981041731e3877bbe0
exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponentjs/exponent,exponentjs/exponent,exponent/exponent,exponent/exponent,exponentjs/exponent