[Bug] in README.md fixtures -> fixture

[Ebazhanov]

Current behavior:

   Inside [README.md](https://github.com/abramenal/cypress-file-upload/blob/master/README.md) file
  for two blocks we have incorrect name of `fixtures`

Desired behavior:

   To match requirements we have to use `fixture` instead of `fixtures`

Steps to reproduce: (app code and test code)

 Use `fixtures` instead of `fixture`

Versions

"cypress-file-upload": "^4.0.7", / macOS / Chrome

FYI

Pull request is read y to go [https://github.com/abramenal/cypress-file-upload/pull/211](https://github.com/abramenal/cypress-file-upload/pull/211)

[josephzidell]

Merged with #211