doc: change term master to leader

[zhjwpku]

What problem does this PR solve?

The term master doesn't match the terminology used in the image and should be kept consistent. Also, master-slave is not the preferred terminology for representing a primary-secondary architecture.

[hello-stephen]

Thank you for your contribution to Apache Doris.
Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[zhjwpku]

Thank you for your contribution to Apache Doris. Don't know what should be done next? See How to process your PR.

Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

I've added a one-line explanation for the changes. I don't think it's necessary to fill out all five points, no?

[zclllyybb]

@morningman PTAL. It do seems there is a mismatch between the text and the image here.