Documentation: Fixed a typo in atomic_t.txt

Fixed a typo in the word 'architecture'.
Signed-off-by: Kushagra Verma <kushagra765@outlook.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/atomic_t.txt

@@ -324,7 +324,7 @@ atomic operations.
 
 Specifically 'simple' cmpxchg() loops are expected to not starve one another
 indefinitely. However, this is not evident on LL/SC architectures, because
-while an LL/SC architecure 'can/should/must' provide forward progress
+while an LL/SC architecture 'can/should/must' provide forward progress
 guarantees between competing LL/SC sections, such a guarantee does not
 transfer to cmpxchg() implemented using LL/SC. Consider: