Fix an alignment issue with the post comments form

[carolinan]

What?

This is the equivalent of #40582 but for the comments form.
Assures that the alignment setting in the block toolbar works for the form elements.
Does not solve the alignment of the comment reply title.

Partial for #40576

Why?

The block had two CSS class names that were missing a single space between them, which meant that the CSS did not apply. Because of this, the alignment option did not work.

How?

By adding a space between the CSS class names.

Testing Instructions

1. Add a posts comment form block in the editor
2. Adjust the alignment from the block tool bar. Duplicate the block and select a different alignment -repeat this for every alignment
3. Save and view the front.
4. Confirm if the alignments in the editor and front match the toolbar setting.

Screenshots or screencast

For the before shot, please see the issue.
With the PR applied:

[cbravobernal]

LGTM!

[gziolo]

Cherry-picked for WordPress 6.0 RC2 release with 5fac2d7.