Add more highlighting to Migration Style Guide doc

[ci skip]

doc/development/migration_style_guide.md

@@ -60,7 +60,7 @@ migration was tested.
 If you need to remove index, please add a condition like in following example:
-```
+```ruby
 remove_index :namespaces, column: :name if index_exists?(:namespaces, :name)
 ```
@@ -75,7 +75,7 @@ need for downtime. To use this method you must disable transactions by calling
 the method `disable_ddl_transaction!` in the body of your migration class like
 so:
-```
+```ruby
 class MyMigration < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -96,7 +96,7 @@ the `up` and `down` methods in your migration class.
 For example, to add the column `foo` to the `projects` table with a default
 value of `10` you'd write the following:
-```
+```ruby
 class MyMigration < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
   disable_ddl_transaction!
@@ -125,7 +125,7 @@ set the limit to 8-bytes. This will allow the column to hold a value up to
 Rails migration example:
-```
+```ruby
 add_column_with_default(:projects, :foo, :integer, default: 10, limit: 8)
 # or
@@ -145,7 +145,7 @@ Please prefer Arel and plain SQL over usual ActiveRecord syntax. In case of usin
 Example with Arel:
-```
+```ruby
 users = Arel::Table.new(:users)
 users.group(users[:user_id]).having(users[:id].count.gt(5))
@@ -154,7 +154,7 @@ users.group(users[:user_id]).having(users[:id].count.gt(5))
 Example with plain SQL and `quote_string` helper:
-```
+```ruby
 select_all("SELECT name, COUNT(id) as cnt FROM tags GROUP BY name HAVING COUNT(id) > 1").each do |tag|
   tag_name = quote_string(tag["name"])
   duplicate_ids = select_all("SELECT id FROM tags WHERE name = '#{tag_name}'").map{|tag| tag["id"]}