Update README.md
it's = it is; grammar correction
Stremio is a full-featured media center designed to help you organize and stream your favorite videos, movies and TV series. It will notify you for new episodes / movies, and allow you to find new content through Discover.
-Stremio allows, using it's [Add-ons system](https://github.com/Stremio/stremio-addon-sdk), to play movies, TV series and channels instantly.
+Stremio allows, using its [Add-ons system](https://github.com/Stremio/stremio-addon-sdk), to play movies, TV series and channels instantly.
## stremio-core
