docs: correct grammar in validation.rst

[eightrix]

Insert 'are' after 'that' to add missing auxilliary verb.

Description
Explain what you have changed, and why.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

@eightrix Thank you!