[docs] Remove reference to inexistent FocusedMark API page

[bernardobelchior]

Fixes https://mui-org.slack.com/archives/C02EQ97NV8F/p1759306642879859.

[mui-bot]

Deploy preview: https://deploy-preview-19773--material-ui-x.netlify.app/

Updated pages:

• docs/data/charts/lines/lines.md

Bundle size report

Bundle	Parsed size	Gzip size
@mui/x-data-grid	0B(0.00%)	0B(0.00%)
@mui/x-data-grid-pro	0B(0.00%)	0B(0.00%)
@mui/x-data-grid-premium	0B(0.00%)	0B(0.00%)
@mui/x-charts	0B(0.00%)	0B(0.00%)
@mui/x-charts-pro	0B(0.00%)	0B(0.00%)
@mui/x-charts-premium	0B(0.00%)	0B(0.00%)
@mui/x-date-pickers	0B(0.00%)	0B(0.00%)
@mui/x-date-pickers-pro	0B(0.00%)	0B(0.00%)
@mui/x-tree-view	0B(0.00%)	0B(0.00%)
@mui/x-tree-view-pro	0B(0.00%)	0B(0.00%)

Details of bundle changes

Generated by 🚫 dangerJS against 59bc306

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #19773 will not alter performance

_{Comparing bernardobelchior:fix-focused-mark-api-page-link (59bc306) with master (7ed4c14)¹}

Summary

✅ 12 untouched

Footnotes

1. No successful run was found on master (d2410cf) during the generation of this report, so 7ed4c14 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩