Add missing $ prompt for bash command examples [ci skip]

Some bash command examples were missing the $ sign.

guides/source/asset_pipeline.md

@@ -30,7 +30,7 @@ and is enabled by default. You can disable it while creating a new application b
 passing the `--skip-sprockets` option.
 
 ```bash
-rails new appname --skip-sprockets
+$ rails new appname --skip-sprockets
 ```
 
 Rails automatically adds the `sass-rails` gem to your `Gemfile`, which is used

guides/source/engines.md

@@ -301,7 +301,7 @@ This means that you will be able to generate new controllers and models for this
 engine very easily by running commands like this:
 
 ```bash
-$ bin/rails g model
+$ bin/rails generate model
 ```
 
 Keep in mind, of course, that anything generated with these commands inside of
@@ -581,7 +581,7 @@ The route now exists, but the controller that this route goes to does not. To
 create it, run this command from the application root:
 
 ```bash
-$ rails g controller comments
+$ rails generate controller comments
 ```
 
 This will generate the following things:
@@ -751,14 +751,14 @@ If you would like to run migrations only from one engine, you can do it by
 specifying `SCOPE`:
 
 ```bash
-rails db:migrate SCOPE=blorgh
+$ rails db:migrate SCOPE=blorgh
 ```
 
 This may be useful if you want to revert engine's migrations before removing it.
 To revert all migrations from blorgh engine you can run code such as:
 
 ```bash
-rails db:migrate SCOPE=blorgh VERSION=0
+$ rails db:migrate SCOPE=blorgh VERSION=0
 ```
 
 ### Using a Class Provided by the Application
@@ -782,7 +782,7 @@ configurable further on). It can be generated using this command inside the
 application:
 
 ```bash
-rails g model user name:string
+$ rails generate model user name:string
 ```
 
 The `rails db:migrate` command needs to be run here to ensure that our

guides/source/generators.md

@@ -473,7 +473,7 @@ class_option :scope, type: :string, default: 'read_products'
 Now our generator can be invoked as follows:
 
 ```bash
-rails generate initializer --scope write_products
+$ rails generate initializer --scope write_products
 ```
 
 The command line arguments are accessed through the `options` method inside the generator class. e.g: