Real-time collaboration: Adjust the WP_ALLOW_COLLABORATION check in writing options.

The `WP_ALLOW_COLLABORATION` constant was introduced in [62075] to support disabling RTC at the host level or at the wp-config.php level. A part of that commit is about hiding the "Enable RTC" checkbox in the writing options when RTC is explicitly disallowed via the constant. This commit fixes an error in the condition to hide that checkbox.

Props zieladam, tyxla.
See #64904.

git-svn-id: https://develop.svn.wordpress.org/trunk@62078 602fd350-edb4-49c9-b593-d223f7449a82