Fix an error in cpp code. (#5189)

Change monster_generate.h to monster_generated.h

docs/source/Tutorial.md

@@ -370,7 +370,7 @@ The first step is to import/include the library, generated files, etc.
 
 <div class="language-cpp">
 ~~~{.cpp}
-  #include "monster_generate.h" // This was generated by `flatc`.
+  #include "monster_generated.h" // This was generated by `flatc`.
 
   using namespace MyGame::Sample; // Specified in the schema.
 ~~~