Use more current platform terminology in README (#419)

diff --git a/README.md b/README.md
index 0d30875..52b9641 100644
--- a/README.md
+++ b/README.md
@@ -4,8 +4,8 @@
 [![Build Status](https://travis-ci.org/dart-lang/http.svg?branch=master)](https://travis-ci.org/dart-lang/http)
 
 This package contains a set of high-level functions and classes that make it
-easy to consume HTTP resources. It's platform-independent, and can be used on
-both the command-line and the browser.
+easy to consume HTTP resources. It's multi-platform, and supports mobile, desktop,
+and the browser.
 
 ## Using