Update README.md

[copocaneta]

PR Checklist

Please check if your PR fulfills the following requirements:

• [X] The commit message follows our guidelines: https://github.com/blockcoders/nestjs-ethers/blob/master/CONTRIBUTING.md
• [X] Tests for the changes have been added (for bug fixes / features)
• [X] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, local variables)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] CI related changes
• [X] Other... Please describe:

README, documentation improvement/update.

What is the current behavior?

this.ethersSigner does not exist on this class. It should be the this.signer from the constructor, right?

What is the new behavior?

It should be the this.signer from the constructor, right?

Does this PR introduce a breaking change?

• [ ] Yes
• [X] No

Other information

Nothing else.

[copocaneta]

Renamed signer variable to ethersSigner to be consistent with other examples