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_pipeline.rs

@@ -630,8 +630,8 @@ impl PhysicsPipeline {
         }
 
         // Finally, make sure we update the world mass-properties of the rigid-bodies
-        // that moved. Otherwise, users may end up applying forces wrt. an outdated
-        // center of mass.
+        // that moved. Otherwise, users may end up applying forces with respect to an
+        // outdated center of mass.
         // TODO: avoid updating the world mass properties twice (here, and
         //       at the beginning of the next timestep) for bodies that were
         //       not modified by the user in the mean time.