Use singular for directory names

exercise/idiomatic/Cargo.toml

@@ -0,0 +1,9 @@
+[package]
+name = "idiomatic"
+version = "0.1.0"
+authors = ["Nathan Stocks <cleancut@github.com>"]
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]