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/include/acpi/acmacros.h b/include/acpi/acmacros.h
index a597207..1954c9d 100644
--- a/include/acpi/acmacros.h
+++ b/include/acpi/acmacros.h
@@ -333,8 +333,8 @@
 #define ACPI_INSERT_BITS(target, mask, source)          target = ((target & (~(mask))) | (source & mask))
 
 /*
- * An struct acpi_namespace_node can appear in some contexts
- * where a pointer to an union acpi_operand_object can also
+ * A struct acpi_namespace_node can appear in some contexts
+ * where a pointer to a union acpi_operand_object can also
  * appear. This macro is used to distinguish them.
  *
  * The "Descriptor" field is the first field in both structures.