Fix typo in error handling section of README

[mickeytgl]

No description provided.

[simonc]

Hi @mickeytgl thanks for opening this PR 😊

Sadly it's only a partial fix as both statements are true, it's sometimes error and sometimes errors (I know… that's 💩)

But nonetheless, you're right, this section should clarify the situation, which mostly boils down to: general errors like auth issue or bad request will get an error key while validation errors like missing template ID will be listed under errors.

[simonc]

@mickeytgl Looking at the code, I realise I actually fixed the API issue in the gem and normalized around document.errors for everything so your PR is correct. My bad! 😅