Typo in class.field.php causes problem with <input> tag

[jamminjames]

Describe the bug
I noticed when viewing the page source of the checkout page when using the plugin that the tags created had no space between "id" and "class", causing the tag to be shown in red in the page source, and possibly causing problems with the results.

By adding a space between the double-quote and single-quote marks in $this->id.'"'; in the classes/class.field.php file at line 521, the problem is fixed:

Was:
$r = '<input name="'.$this->name.'"' .' type="checkbox" value="1"'.' id="'.$this->id.'"';
Fixed:
$r = '<input name="'.$this->name.'"' .' type="checkbox" value="1"'.' id="'.$this->id.'" ';

To Reproduce
Steps to reproduce the behavior:
View the page source of the checkout page when using the plugin

Expected behavior
Input tags should look like:
<input name="transportation" type="checkbox" value="1" id="transportation" class="input " />
...but instead look like:
<input name="transportation" type="checkbox" value="1" id="transportation"class="input " />

WordPress Environment

```Latest WP and plugin, PHP 7.4 ```