Change "Create a post..." to "Write a message..."

web/react/components/create_post.jsx

@@ -248,7 +248,7 @@ module.exports = React.createClass({
                             onUserInput={this.handleUserInput}
                             onKeyPress={this.postMsgKeyPress}
                             messageText={this.state.messageText}
-                            createMessage="Create a post..."
+                            createMessage="Write a message..."
                             channelId={this.state.channel_id}
                             id="post_textbox"
                             ref="textbox" />