fix congratulation message in README.md

README.md

@@ -125,7 +125,7 @@ $ git clone https://github.com/rebar/rebar3
 $ cd rebar
 $ ./bootstrap/bootstrap
 ==> rebar (compile)
-Congratulations! You now have a self-contained script called "rebar" in
+Congratulations! You now have a self-contained script called "rebar3" in
 your current working directory. Place this script anywhere in your path
 and you can use rebar to build OTP-compliant apps.
 ```