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/geometry/collider.rs

@@ -214,7 +214,7 @@ impl Collider {
         &self.pos.0.rotation
     }
 
-    /// The position of this collider wrt the body it is attached to.
+    /// The position of this collider with respect to the body it is attached to.
     pub fn position_wrt_parent(&self) -> Option<&Isometry<Real>> {
         self.parent.as_ref().map(|p| &p.pos_wrt_parent)
     }
@@ -235,7 +235,7 @@ impl Collider {
         }
     }
 
-    /// Sets the position of this collider wrt. its parent rigid-body.
+    /// Sets the position of this collider with respect to its parent rigid-body.
     ///
     /// Does nothing if the collider is not attached to a rigid-body.
     pub fn set_position_wrt_parent(&mut self, pos_wrt_parent: Isometry<Real>) {