Fix a typo in README.md.

The crate is named `ordered-float`, not `ordered_float`.
diff --git a/tutorial/rs/README.md b/tutorial/rs/README.md
index 8ad39ad..166854b 100644
--- a/tutorial/rs/README.md
+++ b/tutorial/rs/README.md
@@ -8,7 +8,7 @@
 
 ```toml
 thrift = "x.y.z" # x.y.z is the version of the thrift compiler
-ordered_float = "0.3.0"
+ordered-float = "0.3.0"
 try_from = "0.2.0"
 ```