Correct README to use onwarn

Seems like it is public API: https://github.com/sveltejs/svelte/blob/master/compiler/index.js#L8.

README.md

@@ -43,7 +43,7 @@ const { code, map } = svelte.compile( source, {
 		console.error( err.message );
 	},
 
-	onwarning: warning => {
+	onwarn: warning => {
 		console.warn( warning.message );
 	}
 });