Add additional instruction in finalise section

What does this MR do?

From this Zendesk ticket(Internal Only), I've learned that we do not explicitly mention the need to escape the , in the arguments obtained from either the logs or the database. If you run a command using arguments from the database or logs without escaping the ,, it will result in this error.

 sudo gitlab-rake gitlab:background_migrations:finalize[CopyColumnUsingBackgroundMigrationJob,merge_request_metrics,id,'[["id"],["id_convert_to_bigint"]]'] --trace
** Invoke gitlab:background_migrations:finalize (first_time)
** Invoke environment (first_time)
** Execute environment
** Execute gitlab:background_migrations:finalize
rake aborted!
JSON::ParserError: unexpected character (after [1]) at line 1, column 7 [parse.c:733] in '[["id"]
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/json.rb:107:in `rescue in adapter_load'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/json.rb:102:in `adapter_load'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/json.rb:28:in `parse'
/opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/background_migrations.rake:76:in `finalize_migration'
/opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/background_migrations.rake:18:in `block (3 levels) in <top (required)>'
/opt/gitlab/embedded/bin/bundle:23:in `load'
/opt/gitlab/embedded/bin/bundle:23:in `<main>'

Caused by:
EncodingError: unexpected character (after [1]) at line 1, column 7 [parse.c:733] in '[["id"]
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/json.rb:105:in `load'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/json.rb:105:in `adapter_load'
/opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/json.rb:28:in `parse'
/opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/background_migrations.rake:76:in `finalize_migration'
/opt/gitlab/embedded/service/gitlab-rails/lib/tasks/gitlab/background_migrations.rake:18:in `block (3 levels) in <top (required)>'
/opt/gitlab/embedded/bin/bundle:23:in `load'
/opt/gitlab/embedded/bin/bundle:23:in `<main>'
Tasks: TOP => gitlab:background_migrations:finalize

So by adding a backslash (\) before the command resolves this

Example.

For database results, here

job_class_name                                 |         table_name          | column_name |                     job_arguments                      | status 

-----------------------------------------------+-----------------------------+-------------+--------------------------------------------------------+--------
 CopyColumnUsingBackgroundMigrationJob         | merge_request_metrics       | id          | [["id"], ["id_convert_to_bigint"]]                     |      1

The command should be sudo gitlab-rake gitlab:background_migrations:finalize[CopyColumnUsingBackgroundMigrationJob,merge_request_metrics,id,'[["id"]\,["id_convert_to_bigint"]]'] --trace

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