fix: typo of sign up button in Register component

Author: frankOnTheWay

src/components/Register.js

@@ -96,7 +96,7 @@ class Register extends React.Component {
                     className="btn btn-lg btn-primary pull-xs-right"
                     type="submit"
                     disabled={this.props.inProgress}>
-                    Sign in
+                    Sign up
                   </button>
 
                 </fieldset>