Renamed add_children to add_child since it adds only one child (#6838)

substrate/client/service/src/task_manager/mod.rs

@@ -344,7 +344,7 @@ impl TaskManager {
 	/// Register another TaskManager to terminate and gracefully shutdown when the parent
 	/// terminates and gracefully shutdown. Also ends the parent `future()` if a child's essential
 	/// task fails. (But don't end the parent if a child's normal task fails.)
-	pub fn add_children(&mut self, child: TaskManager) {
+	pub fn add_child(&mut self, child: TaskManager) {
 		self.children.push(child);
 	}
 }