Update forms.md

[adamklepacz]

I have changed classes in #### Checkboxes and Radio Buttons cause in yours HTML EXAMPLE third checkbox was collapsing to the next line. That may was not understandable for some users.
Now it displays proper

Description

I have changed classes in #### Checkboxes and Radio Buttons cause in yours HTML EXAMPLE third checkbox was collapsing to the next line.

• Closes

Motivation and Context

Change is about better user experience. Third checkbox was collapsing to the next line so it may was not understandable to some users.

Screenshots (if appropriate):

https://cdn3.imggmi.com/uploads/2018/5/20/0d0169ab644d95fe8dfe3e904d97f86b-full.png - after changes
https://cdn3.imggmi.com/uploads/2018/5/20/377786353be7cb3150513c91f3d83972-full.png - before changes

Types of changes

Change of classes inside HTML example.

• Documentation
• 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 change)

Checklist (all required):

• [x ] I have read and follow the CONTRIBUTING document.
• [x ] There are no other pull request similar to this one.
• [x ] The pull request title is descriptive.
• [x ] The template is fully and correctly filled.
• [x ] The pull request targets the right branch (develop or support/*).
• [x ] My commits are correctly titled and contain all relevant information.
• [ x] My code follows the code style of this project.
• [x ] I have updated the documentation accordingly to my changes (if relevant).
• I have added tests to cover my changes (if relevant).
• [x ] All new and existing tests passed.

[ncoden]

LGTM 👍

@adamklepacz Please note that all pull requests must be opened on develop now, even for documentation changes. I took care of this.

See CONTRIBUTING.md