Fix duplicated heading ID in the Button docs

If the users click on Link in the menu they were redirected
to the Button Link subsection instead because it has the same
autogenerated ID.

To avoid such problems we should try to keep all headings
and subheading different within all the files. Despite it's better
to find another solution.

src/elements/button/button.md

@@ -8,7 +8,7 @@
 <Button variant="text">Text</Button>
 ```
 
-### Link
+### Button as a link
 
 ```jsx
 <Button variant="contained" href="#" tag="a">Next</Button>