doc: Fix alt text in auto_merge.md

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do and why?

Related to: #479161

Improves alt text for the merge pipeline cancel option to be more descriptive and accessible.

Changes made:

• Updated alt text from "Status" to "The cancel merge when pipeline succeeds option"

Before:

![Status](img/cancel-mwps_v15_4.png)

After:

![The cancel merge when pipeline succeeds option](img/cancel-mwps_v15_4.png)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.