[BrokenLinksH2] fixing broken links

[jdelauri]

Fixed download link in line 23 and link in line 256

[opbld34]

Docs Build status updates of commit dccde76:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/how-to-add-custom-ui-to-a-spreadsheet-document.md	✅Succeeded

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[jdelauri]

fixed download link and link in line 256

[jdelauri]

@tfosmark please review and merge

[jdelauri]

In article Welcome to the Open XML SDK 2.5 for Office | Microsoft Docs, there’s a See Also link that’s similar (Open XML SDK 2.5 for Microsoft Office) so I modified the broken link to point to the same target.

[tfosmark]

@lindalu-MSFT

Global effort to fix broken links

The Content & Learning team is fixing broken links on docs.microsoft.com for the rest of H2. This effort will eliminate potential accessibility, security, and usability issues. This PR includes only link fixes and does not change other content. Please review and merge, or comment in the PR with any changes you'd like to see. Thanks!