Update return types in docs for Mix.Generator functions, closes #9749

Author: José Valim

lib/mix/lib/mix/generator.ex

@@ -18,7 +18,7 @@ defmodule Mix.Generator do
 
       iex> Mix.Generator.create_file(".gitignore", "_build\ndeps\n")
       * creating .gitignore
-      :ok
+      true
 
   """
   @spec create_file(Path.t(), iodata, keyword) :: boolean()
@@ -48,7 +48,7 @@ defmodule Mix.Generator do
 
       iex> Mix.Generator.create_directory("path/to/dir")
       * creating path/to/dir
-      :ok
+      true
 
   """
   @spec create_directory(Path.t(), keyword) :: true
@@ -73,7 +73,7 @@ defmodule Mix.Generator do
 
       iex> Mix.Generator.copy_file("source/gitignore", ".gitignore", "_build\ndeps\n")
       * creating .gitignore
-      :ok
+      true
 
   """
   @doc since: "1.9.0"
@@ -99,7 +99,7 @@ defmodule Mix.Generator do
 
       iex> Mix.Generator.copy_template("source/gitignore", ".gitignore", [project_path: path])
       * creating .gitignore
-      :ok
+      true
 
   """
   @doc since: "1.9.0"