docs: Expand "wrt" to "with respect to".

This makes things more clear as this abbreviation isn't known
to everyone.

While the trailing period is common in French, it isn't in English
(one might use "w.r.t.").

Fixes #498.

src/pipeline/physics_hooks.rs

@@ -95,7 +95,8 @@ impl<'a> ContactModificationContext<'a> {
             CONTACT_CURRENTLY_FORBIDDEN => {
                 // Contacts are forbidden so we need to continue forbidding contacts
                 // until all the contacts are non-penetrating again. In that case, if
-                // the contacts are OK wrt. the contact normal, then we can mark them as allowed.
+                // the contacts are OK with respect to the contact normal, then we can
+                // mark them as allowed.
                 if contact_is_ok && self.solver_contacts.iter().all(|c| c.dist > 0.0) {
                     *self.user_data = CONTACT_CURRENTLY_ALLOWED;
                 } else {