docs: updated link to default theme (#55)

The theme file that was being linked to in the documentation is incorrect.

Author: cereallarceny

docs/basics/Theming.mdx

@@ -46,11 +46,11 @@ render(
 
 ## Default theme
 
-You can find all theme values in [the default theme](https://github.com/smooth-code/smooth-ui/blob/master/src/theme.js).
+You can find all theme values in [the default theme](https://github.com/smooth-code/smooth-ui/blob/master/packages/shared/core/theme.js).
 
 Two ways to import theme in your code:
 
 ```js
 import { theme } from '@smooth-ui/core-sc'
 import theme from '@smooth-ui/core-sc/theme'
-```
+```