add missing closing quote to name attribute for Triangle plugin. (ros2#5842)

Signed-off-by: Tomoya.Fujita <[email protected]>

Author: fujitatomoya

source/Tutorials/Beginner-Client-Libraries/Pluginlib.rst

@@ -222,7 +222,7 @@ Create ``~/ros2_ws/src/polygon_plugins/plugins.xml`` with the following code:
       <class type="polygon_plugins::Square" base_class_type="polygon_base::RegularPolygon">
         <description>This is a square plugin.</description>
       </class>
-      <class type="polygon_plugins::Triangle" base_class_type="polygon_base::RegularPolygon" name="awesome_triangle>
+      <class type="polygon_plugins::Triangle" base_class_type="polygon_base::RegularPolygon" name="awesome_triangle">
         <description>This is a triangle plugin.</description>
       </class>
     </library>