Update merge request diff navigation docs

Update screenshot for merge request diff navigation.

Related issues

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/14249

Author's checklist

• Apply the correct labels and milestone
• [ ] Crosslink the document from the higher-level index
• [ ] Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• [ ] Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards