upercase PORT in usage

README.md

@@ -8,7 +8,7 @@ a simple http server like `python -m http.server` but:
 
 ```
 USAGE:
-    http-server [OPTIONS] [port]
+    http-server [OPTIONS] [PORT]
 
 FLAGS:
     -h, --help       Prints help information
@@ -19,7 +19,7 @@ OPTIONS:
         --chdir <DIRECTORY>    Specify directory to server [default: .]
 
 ARGS:
-    <port>    Specify alternate port [default: 8000]
+    <PORT>    Specify alternate port [default: 8000]
 ```
 
 ## FAQ

src/main.rs

@@ -36,6 +36,7 @@ fn main() -> Result<(), io::Error> {
                     .default_value("0.0.0.0")
                     .takes_value(true))
                 .arg(Arg::with_name("port")
+                    .value_name("PORT")
                     .help("Specify alternate port")
                     .default_value("8000")
                     .index(1));