docs: Fix documentation errors (iluwatar#2337)

Author: wulangcode

visitor/README.md

@@ -50,11 +50,11 @@ public abstract class Unit {
 
 public interface UnitVisitor {
 
-  void visitSoldier(Soldier soldier);
+  void visit(Soldier soldier);
 
-  void visitSergeant(Sergeant sergeant);
+  void visit(Sergeant sergeant);
 
-  void visitCommander(Commander commander);
+  void visit(Commander commander);
 }
 ```