Documentation/memory-barriers.txt: Fix a typo in the data dependency description

This typo has been there forever, it is 7.5 years old, looks like this
section of our memory ordering documentation is a place where most eyes
are glazed over already ;-)

[ Also fix some stray spaces and stray tabs while at it, shrinking the
  file by 49 bytes. Visual output unchanged. ]

Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/n/tip-gncea9cb8igosblizfqMXrie@git.kernel.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>

Documentation/memory-barriers.txt

@@ -500,7 +500,7 @@ odd-numbered bank is idle, one can see the new value of the pointer P (&B),
 but the old value of the variable B (2).
 
 
-Another example of where data dependency barriers might by required is where a
+Another example of where data dependency barriers might be required is where a
 number is read from memory and then used to calculate the index for an array
 access: