readme: link only runs on master from status badge

By default the badge displays the status only for the default branch,
but we linked to all runs including for pull requests when clicking
on the badge.
Also the URL for the icon differed from GitHub's documentation.

Author: TheOneric

README.md

@@ -1,4 +1,5 @@
-[![Actions Status](https://github.com/libass/JavascriptSubtitlesOctopus/workflows/Emscripten/badge.svg)](https://github.com/libass/JavascriptSubtitlesOctopus/actions)
+[![Actions Status](https://github.com/libass/JavascriptSubtitlesOctopus/actions/workflows/emscripten.yml/badge.svg)](https://github.com/libass/JavascriptSubtitlesOctopus/actions/workflows/emscripten.yml?query=branch%3Amaster+event%3Apush)
+
 
 SubtitlesOctopus displays subtitles in .ass format and easily integrates with HTML5 videos.
 Since it uses [libass](https://github.com/libass/libass), SubtitlesOctopus supports most