Small error in the ReadMe

[ethan2k]

There's a small error in the ReadMe where it is not matching the actual code:

It shows as examples the usage of:

errors.AddContext(err, "field", "message")
errors.GetContext(err) // map[string]string{"field": "field", "message": "message"}

But in the code, the methods are actually named respectively:

errors.AddErrorContext(...)
errors.GetErrorContext(...)

Last but certainly not least: this looks like a great example. Thank you for the inspiration!