Remove extra newline from APIDiff TOC

[ericstj]

Fixes #49793

Each link is already added with a trailing newline:

tableOfContents.AppendLine($"* [{assemblyName}]({fileName})");

Adding another new-line makes an empty line at the end of the file that's triggering the linter. Remove this.

[Copilot]

Pull Request Overview

This PR removes an unnecessary blank line appended to the generated API diff table of contents, which was triggering the linter for trailing empty lines.

• Eliminates the extra AppendLine() call so each entry’s existing newline is sufficient.

[tarekgh]

@ericstj looks we need to fix the tests https://helixr1107v0xdeko0k025g8.blob.core.windows.net/dotnet-sdk-refs-pull-49794-merge-89c633b9d6fb4aacb6/Microsoft.DotNet.ApiDiff.Tests.dll.3/3/console.c1cfa216.log?helixlogtype=result