Update Yarn mappings to 1.14.1+build.5

Yarn 1.14.1+build.4 had an issue with Hopper#getWorld being renamed to Hopper#getHopperWorld, which caused the world to crash when ticking hoppers in dev. 1.14.1+build.5 fixed this, but the example mod wasn't updated yet, so anyone following the dev install instructions on the site would run into the issue.

gradle.properties

@@ -4,7 +4,7 @@ org.gradle.jvmargs=-Xmx1G
 # Fabric Properties
 	# check these on https://fabricmc.net/use
 	minecraft_version=1.14.1
-	yarn_mappings=1.14.1+build.4
+	yarn_mappings=1.14.1+build.5
 	loader_version=0.4.7+build.147
 
 # Mod Properties