docs: gcov: fix link to LCOV website

The previous website hosted on SourceForge is no longer available since
January 2024 according to archive.org [1].

It looks like the website has been officially moved to GitHub in June
2022 [2]. Best to redirect readers to the new location then.

Link: https://web.archive.org/web/20240105235756/https://ltp.sourceforge.net/coverage/lcov.php [1]
Link: https://github.com/linux-test-project/lcov/commit/6da8399c7a7a

 [2]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Acked-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240926-doc-fix-lcov-link-v1-1-46f250cb7173@kernel.org