doc: fix some typos in documentations

Fix some typos in five documentations, no functional change.

Signed-off-by: Xishi Qiu <qiuxishi@huawei.com>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/Documentation/rt-mutex-design.txt b/Documentation/rt-mutex-design.txt
index a5bcd7f..8666070 100644
--- a/Documentation/rt-mutex-design.txt
+++ b/Documentation/rt-mutex-design.txt
@@ -30,7 +30,7 @@
 priority process is prevented from running by a lower priority process for
 an undetermined amount of time.
 
-The classic example of unbounded priority inversion is were you have three
+The classic example of unbounded priority inversion is where you have three
 processes, let's call them processes A, B, and C, where A is the highest
 priority process, C is the lowest, and B is in between. A tries to grab a lock
 that C owns and must wait and lets C run to release the lock. But in the