Changed 'skip' wording

As outlined in <https://webaim.org/techniques/skipnav/>

src/components/Toolbar.jsx

@@ -140,7 +140,7 @@ export default class Toolbar extends React.Component {
           className="maputnik-toolbar-logo"
         >
           <a tabIndex="1" className="maputnik-toolbar-skip" href="#skip-menu">
-            Skip the menu
+            Skip navigation
           </a>
           <img src={logoImage} alt="Maputnik" />
           <h1>Maputnik