changed dmd-doc.desktop Exec line.

[CJPrindle]

The dmd-doc.desktop file is pointing to the wrong html doc page.
New Path: Exec=xdg-open usr/share/dmd/html/d/spec/spec.html

[dlang-bot]

Thanks for your pull request and interest in making D better, @CJPrindle! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the annotated coverage diff directly on GitHub with CodeCov's browser extension
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

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If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.