fsverity.rst: update git repo URL for fsverity-utils

The fsverity-utils git repo is moving out of my personal directory;
see the announcement at
https://lore.kernel.org/r/Y9GKm+hcm70myZkr@sol.localdomain

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Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20230125203025.28187-1-ebiggers@kernel.org