Fix readme header

diff --git a/README.md b/README.md
index b9daf51..b62b447 100644
--- a/README.md
+++ b/README.md
@@ -1,4 +1,4 @@
-###A Dart 2D physics engine
+### A Dart 2D physics engine
 
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 [![Coverage Status](https://coveralls.io/repos/google/box2d.dart/badge.svg?branch=master)](https://coveralls.io/r/google/box2d.dart)