Revert "Don't suggest use of usize instead of u32"
Reason for revert: the types u64 and u128 are not usable as graph
indices, so we can't suggest that users use them.
This reverts commit a535249a054f23decaaace360ddad7a9625f0189.
diff --git a/src/graph_impl/mod.rs b/src/graph_impl/mod.rs
index d3c7d1b..dc8371d 100644
--- a/src/graph_impl/mod.rs
+++ b/src/graph_impl/mod.rs
@@ -303,7 +303,7 @@
/// take its place. Similarly, removing an edge shifts the index of the last edge.
///
/// The `Ix` parameter is `u32` by default. The goal is that you can ignore this parameter
-/// completely unless you need a very big graph -- then you can use `u64` or `u128`.
+/// completely unless you need a very big graph -- then you can use `usize`.
///
/// ### Pros and Cons of Indices
///
