docs: update exec command with namespace

What does this MR do?

What

Update the exec command to specify the namespace to connect to toolbox

Why

Following the current instructions:

$ kubectl --namespace gitlab get pods -lapp=toolbox
NAME                                       READY   STATUS      RESTARTS   AGE
ops-gitlab-toolbox-6f7487f9d6-4tvjv        1/1     Running     0          2d7h
ops-gitlab-toolbox-backup-28363740-nvjc2   0/1     Completed   0          32h
ops-gitlab-toolbox-backup-28365180-t8gzr   0/1     Completed   0          8h
$ kubectl exec -it ops-gitlab-toolbox-6f7487f9d6-4tvjv -- bash
Error from server (NotFound): pods "ops-gitlab-toolbox-6f7487f9d6-4tvjv" not found

We need to specify --namespace gitlab

$ kubectl exec --namespace gitlab -it ops-gitlab-toolbox-6f7487f9d6-4tvjv -- bash
Defaulted container "toolbox" out of: toolbox, certificates (init), configure (init)

Author's checklist

• Ensure the branch name starts with docs- or ends with -docs, so only the docs-related CI jobs are included
• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.