Documentation: trace/stm: drop doubled words

Change "and and" to "and an".
Fix spello of "example".

Signed-off-by: Randy Dunlap <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Cc: Steven Rostedt <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/trace/stm.rst

···
 have a name (string identifier) and a range of masters and channels
 associated with it, located in "stp-policy" subsystem directory in
 configfs. The topmost directory's name (the policy) is formatted as
-the STM device name to which this policy applies and and arbitrary
-string identifier separated by a stop. From the examle above, a rule
+the STM device name to which this policy applies and an arbitrary
+string identifier separated by a stop. From the example above, a rule
 may look like this::
 
 	$ ls /config/stp-policy/dummy_stm.my-policy/user