docs: add missing export on concepts/context (#1196)

Author: xyaman

src/routes/concepts/context.mdx

@@ -24,7 +24,7 @@ This function has a `Provider` property that wraps the component tree you want t
 ```jsx tab title="/context/create.js"
 import { createContext } from "solid-js";
 
-const MyContext = createContext();
+export const MyContext = createContext();
 ```
 
 ```jsx tab title="/context/component.jsx"