trivial: fix an -> a typos in documentation and comments

It is always "an" if there is a vowel _spoken_ (not written).
So it is:
"an hour" (spoken vowel)
but
"a uniform" (spoken 'j')

Signed-off-by: Frederik Schwarzer <schwarzerf@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/acpi/executer/exstore.c b/drivers/acpi/executer/exstore.c
index 3318df4..1c118ba 100644
--- a/drivers/acpi/executer/exstore.c
+++ b/drivers/acpi/executer/exstore.c
@@ -274,7 +274,7 @@
  *
  * PARAMETERS:  *source_desc        - Value to be stored
  *              *dest_desc          - Where to store it.  Must be an NS node
- *                                    or an union acpi_operand_object of type
+ *                                    or a union acpi_operand_object of type
  *                                    Reference;
  *              walk_state          - Current walk state
  *