Add clarification to steps for Kubernetes Agent install

What does this MR do?

Clarifies which value from the json output of a previous step needs to be used in the current step.

The comment line also says "...you need to use in the next step" but there is at least one step in between (kubectl create namespace gitlab-kubernetes-agent) or it is unclear what a "step" is.

NOTE: The screenshot is also missing data compared to actual outputs. It would be better to include the output and obscure it or simply delete the token in it. It would be much easier to understand the secret value when you see one on the output.

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.