Update README.md payload & error types

[dcecile]

Implementations:

sorbet-result/lib/typed/result.rb

Lines 104 to 106 in 86319a4

	def error
	raise NoErrorOnSuccessError
	end

sorbet-result/lib/typed/result.rb

Lines 179 to 181 in 86319a4

	def payload
	raise NoPayloadOnFailureError
	end

[maxveldink]

Thank you!

[dcecile]

Oh! I just read the diff again right now, and there's also a typo on the line I changed: #76