docs: locking: Add 'need' to hardirq section

Add the missing word to make this sentence read properly.

Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/20200318174133.160206-2-swboyd@chromium.org


Signed-off-by: Jonathan Corbet <corbet@lwn.net>