fixing the readme to indicate err return

from a test that I wrote in https://github.com/arschles/go-bindata-test, it looks like go-bindata generates an Asset(...) function that returns an error when the file isn't found

README.md

@@ -56,8 +56,8 @@ To ignore files, pass in regexes using -ignore, for example:
 To access asset data, we use the `Asset(string) []byte` function which
 is included in the generated output.
 
-	data := Asset("pub/style/foo.css")
-	if len(data) == 0 {
+	data, err := Asset("pub/style/foo.css")
+	if err != nil {
 		// Asset was not found.
 	}