[UIK-2393][docs, form] fixed form validation examples with incorrect validation patterns

[ksimanov]

Motivation and Context

Fixed form validation examples with incorrect validation patterns

How has this been tested?

manually

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).
• Nice improve.

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly or it's not required.
• Unit tests are not broken.
• I have added changelog note to corresponding CHANGELOG.md file with planned publish date.
• I have added new unit tests on added of fixed functionality.