docs: fixup a whitespace error

The patch fixes a whitespace error in locking guidelenes document.

Change-Id: I7b704ef7cf5ddca7e67e96e727b6ff06d7092fea
Signed-off-by: Soby Mathew <soby.mathew@arm.com>
diff --git a/docs/design/locking.rst b/docs/design/locking.rst
index 7ac2151..09cb292 100644
--- a/docs/design/locking.rst
+++ b/docs/design/locking.rst
@@ -532,7 +532,7 @@
 	- DEV_GRANULE_STATE_DELEGATED
 
 #. **Internal**: A granule state belongs to the `internal` class iff it is not
-   an `external`. These are objects which are referenced from another
+   an `external`. These are objects which are referenced from another
    object after that object is locked. Each `internal` object should be
    referenced from exactly one place. The following granule states are
    `internal`: