Fix theming unsubscribe type

Author: lttb

src/styled.js

@@ -36,7 +36,7 @@ const styled = ({tagName, elementStyle, mountSheet}: StyledArgs) => {
     props: StyledElementPropsType
     sheet: JssSheet
     state: {theme?: Object}
-    unsubscribe: Function
+    unsubscribe: ?Function
 
     dynamicTagName = ''
     setTheme = (theme: Object) => this.setState({theme})