Update CAPTCHA labels to reflect the symbols in the CAPTCHA image

[RhodriOwainDavies]

Description

The text labels referring to the CAPTCHA image only refer to letters, when there are also numbers in the CAPTCHA images. Meaning the instruction to the user is ambiguous. The message implies to NOT enter the numbers, thus the CAPTCHA doesn't match.

I've changed the labels in a few places and updated the dictionary file

Manual testing scenarios

1. As a customer visit the sign-in page and enter the wrong password for the same email address several times until the captcha image & label appears
2. As a merchant visit the admin login page and enter the wrong password for the same user account several times until the captcha image & label appears
3. Install a language pack which includes a dictionary key for the new phrases, change the language of the store and repeat steps 1 & 2 ensuring the phrases appear in their translated forms.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)