update yarn

gradle.properties

@@ -5,7 +5,7 @@ org.gradle.jvmargs=-Xmx1G
 	# check these on https://fabricmc.net/use
     # or better: https://modmuss50.me/fabric.html
 	minecraft_version=1.15.1
-	yarn_mappings=1.15.1+build.18
+	yarn_mappings=1.15.1+build.28
 	loader_version=0.7.3+build.176
 
 # Mod Properties