Docs: remove redundant space in forms/validation.md

[wxh06]

Description

Remove redundant space in forms/validation.md

-      <input type="text" class="form-control" id="validationDefaultUsername"  aria-describedby="inputGroupPrepend2" required>
+      <input type="text" class="form-control" id="validationDefaultUsername" aria-describedby="inputGroupPrepend2" required>

Motivation & Context

I've been seeing this space for so long :(

Type of changes

Typo fix in docs

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-37994--twbs-bootstrap.netlify.app/

Related issues