readme: update

README.md

@@ -9,11 +9,11 @@ A simple CLI library for Carp.
   (let [p (=> (CLI.new @"My super cool tool!")
               (CLI.add &(CLI.option "--flag" "-f" "my flag" true)))]
     (match (CLI.parse &p)
-      (Result.Success p) (IO.println &(str &(CLI.get &p "--flag")))
+      (Result.Success p) (IO.println &(str &(CLI.get &p "flag")))
       (Result.Error msg) (do (IO.errorln &msg) (CLI.usage &p)))))
 ```
 
-## Usage
+## Installation
 
 ```clojure
 (load "git@github.com:carpentry-org/cli.carp.git@master")