Add dot a the end of sentence per code review

Co-authored-by: Tomoya Fujita <Tomoya.Fujita@sony.com>
Signed-off-by: Michael Orlov <morlovmr@gmail.com>

Author: MichaelOrlov

source/The-ROS2-Project/Contributing/Code-Style-Language-Versions.rst

@@ -163,7 +163,7 @@ Doxygen Comments
 ~~~~~~~~~~~~~~~~
 
 * While it is allowed to use ``/** */`` style comments for documentation, we prefer ``///`` style
-  comments for Doxygen
+  comments for Doxygen.
 
   * rationale: ``///`` is more consistent with the rest of the C++ code
   * rationale: ``///`` is easier to type and read in most editors