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/data/graph.rs

@@ -1,5 +1,5 @@
 // This is basically a stripped down version of petgraph's UnGraph.
-// - It is not generic wrt. the index type (we always use u32).
+// - It is not generic with respect to the index type (we always use u32).
 // - It preserves associated edge iteration order after Serialization/Deserialization.
 // - It is always undirected.
 //! A stripped-down version of petgraph's UnGraph.