fix: README injected variable ethersContract

[GustavoRSSilva]

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

PR Type

Simple documentation fix to enable copy-pasting the read me.

What is the current behavior?

Wrong description

Issue Number: N/A

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

[0xslipk]

@GustavoRSSilva thanks for the PR. I really appreciate it.