docs: update UPGRADE.md

Author: mrexox

UPGRADE.md

@@ -1,9 +1,11 @@
 # v1 -> v2
 
-Notable changes:
+## Notable changes
 
 - **v2** uses [universal coverage reporter](https://github.com/coverallsapp/coverage-reporter) instead of [node-coveralls](https://github.com/nickmerwin/node-coveralls) like in **v1**.
 
+## Upgrading
+
 - Option `path-to-lcov` is now deprecated, use `file` instead.
 
 - You can skip `file` option and coveralls will try to find all supported coverage files and combine their data.