init: fix typo and prevent startup error

metrics_library upstart job failes on boot because of a typo and soft
link command linking on existing file. Fixing the typo and add '-f' flag
when linking to supress error.

BUG=chrome-os-partner:40606
TEST=`start metrics_library`

Change-Id: I3c144f4a636a61bb460a9de74771f61ad1e6b0f9
Reviewed-on: https://chromium-review.googlesource.com/273172


Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Wei-Ning Huang <wnhuang@chromium.org>
Tested-by: Wei-Ning Huang <wnhuang@chromium.org>