Update heron.md

projects/robots/clearpath/heron/protos/docs/heron.md

@@ -2,7 +2,7 @@ The [clearpath's Heron USV](https://robots.ros.org/clearpath-heron-usv/) is a US
 
 ### Motors
 
-There are 2 motor, motor name ```left_motor```and ```right_motor``.
+There are 2 motor, motor name ```left_motor```and ```right_motor```.
 You can add new Camera, GPS or other sensors you want under ```body_slot```.
 
 ### Movie Presentation