Update Exercises Directory Names to Reflect Order

exercises/09_strings/README.md

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+# Strings
+
+Rust has two string types, a string slice (`&str`) and an owned string (`String`).
+We're not going to dictate when you should use which one, but we'll show you how
+to identify and create them, as well as use them.
+
+## Further information
+
+- [Strings](https://doc.rust-lang.org/book/ch08-02-strings.html)