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

Documentation/kernel-hacking/locking.rst

@@ -263,7 +263,7 @@ by a hardware interrupt on another CPU. This is where
 interrupts on that cpu, then grab the lock.
 :c:func:`spin_unlock_irq()` does the reverse.
 
-The irq handler does not to use :c:func:`spin_lock_irq()`, because
+The irq handler does not need to use :c:func:`spin_lock_irq()`, because
 the softirq cannot run while the irq handler is running: it can use
 :c:func:`spin_lock()`, which is slightly faster. The only exception
 would be if a different hardware irq handler uses the same lock: