Fixed typo in README examples

[tomaj]

Pull request type

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Other (please describe):

How to test this PR

no need to test. it just typo in README

What is the new behavior?

proper documentation in README file

Other information

n/a

[roberto-butti]

Thank you, @tomaj, for the PR. You had a good eye.

[roberto-butti]

@tomaj I'm going to merge this PR; I see you are a first-time contributor for Storyblok PHP Client! Welcome!