fix typo in Real-Time-Programming.rst (ros2#4501) (ros2#4507)

Signed-off-by: Christian Flewelling <[email protected]>
(cherry picked from commit b0f5ec5)

Co-authored-by: Christian Flewelling <[email protected]>

Author: mergify[bot]

source/Tutorials/Demos/Real-Time-Programming.rst

@@ -35,7 +35,7 @@ The real-time demo was written with Linux operating systems in mind, since many
 Since many of the operations done in the demo to optimize performance are OS-specific, the demo only builds and runs on Linux systems.
 **So, if you are an OSX or Windows user, don't try this part!**
 
-Also this must be built from source using a the static DDS API. **Currently the only supported implementation is Connext**.
+Also this must be built from source using a static DDS API. **Currently the only supported implementation is Connext**.
 
 First, follow the instructions to build ROS 2 :doc:`from source <../../Installation/Alternatives/Ubuntu-Development-Setup>` using Connext DDS as the middleware.