Fix type in 0.13 blog post

Pull in #2971 to the markdown

docs/_posts/2015-01-27-react-v0.13.0-beta-1.md

@@ -33,7 +33,7 @@ class HelloMessage extends React.Component {
 React.render(<HelloMessage name="Sebastian" />, mountNode);
 ```
 
-The API is mostly what you would expect, which the exception for `getInitialState`. We figured that the idiomatic way to specify class state is to just use a simple instance property. Likewise `getDefaultProps` and `propTypes` are really just properties on the constructor.
+The API is mostly what you would expect, with the exception for `getInitialState`. We figured that the idiomatic way to specify class state is to just use a simple instance property. Likewise `getDefaultProps` and `propTypes` are really just properties on the constructor.
 
 ```javascript
 export class Counter extends React.Component {