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) <[e-mail]>
Acked-by: Peter Oberparleiter <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/dev-tools/gcov.rst

···
   associated code is never run?)
 
 .. _gcov: https://gcc.gnu.org/onlinedocs/gcc/Gcov.html
-.. _lcov: http://ltp.sourceforge.net/coverage/lcov.php
+.. _lcov: https://github.com/linux-test-project/lcov
 
 
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