README.md: fix emoji translation link

The upstream has renamed master into main and clicking the old link
takes you to "old_master" with a message

> Branch master was renamed to old_master.

README.md

@@ -182,4 +182,4 @@ location /_matrix/push/v1/notify {
 
 * <a href="https://github.com/googlefonts/noto-emoji/">Noto Emoji Font</a> for the awesome emojis.
 
-* The Matrix Foundation for making and maintaining the [emoji translations](https://github.com/matrix-org/matrix-doc/blob/master/data-definitions/sas-emoji.json) used for emoji verification, licensed Apache 2.0
+* The Matrix Foundation for making and maintaining the [emoji translations](https://github.com/matrix-org/matrix-doc/blob/main/data-definitions/sas-emoji.json) used for emoji verification, licensed Apache 2.0