En-es library update

[fco-acuna]

Motivation / Background

This Pull Request has been created because I saw some errors and missing terms in the EN-MX library and I wanted to fix it and contribute to the gem.

Additional information

I´m from Mexico, my first language is Spanish. I thought I could correct some terms in the EN-MX library according to what I know is more frequently used here in Mexico.

Checklist

Before submitting the PR make sure the following are checked:

• [ ] This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• [ ] Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• [ ] Tests are added or updated if you fix a bug, refactor something, or add a feature.
• [ ] Tests and Rubocop are passing before submitting your proposed changes.

If you're proposing a new generator or locale:

• [x] Double-check the existing generators documentation to make sure the new generator you want to add doesn't already exist.
• [x] You've reviewed and followed the Contributing guidelines.