pkg/shelf: spelling fixes

README.md

 ## Web Server Middleware for Dart
 
 [![Build Status](https://travis-ci.org/dart-lang/shelf.svg?branch=master)](https://travis-ci.org/dart-lang/shelf)
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 ## Introduction
 
 **Shelf** makes it easy to create and compose **web servers** and **parts of web
 servers**. How?
 
 * Expose a small set of simple types.
 * Map server logic into a simple function: a single argument for the request,
 the response is the return value.
 * Trivially mix and match synchronous and asynchronous processing.
-* Flexibliity to return a simple string or a byte stream with the same model.
+* Flexibility to return a simple string or a byte stream with the same model.
 
 ## Example
 
 See `example/example_server.dart`
 
 ```dart
 import 'package:shelf/shelf.dart' as shelf;
 import 'package:shelf/shelf_io.dart' as io;
 
 void main() {
@@ skipping 122 matching lines @@
 }
 ```
 
 ## Inspiration
 
 * [Connect](http://www.senchalabs.org/connect/) for NodeJS.
     * Read [this great write-up](http://howtonode.org/connect-it) to understand
       the overall philosophy of all of these models.
 * [Rack](http://rack.github.io/) for Ruby.
 * [WSGI](http://legacy.python.org/dev/peps/pep-3333/) for Python.