Fix typo in styled-components import (#267)

Author: ssnielsen

Diff for: docs/docs/styled-styling.mdx

@@ -14,7 +14,7 @@ The Component support styling using `Styled-component`:
 ```jsx
 import React from 'react';
 import Popup from 'reactjs-popup';
-import Styled from 'styled-components';
+import styled from 'styled-components';
 
 const StyledPopup = styled(Popup)`
   // use your custom style for ".popup-overlay"