scripts/spelling.txt: add "an union" pattern and fix typo instances

Fix typos and add the following to the scripts/spelling.txt:

  an union||a union

Link: http://lkml.kernel.org/r/1481573103-11329-5-git-send-email-yamada.masahiro@socionext.com
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/acpi/acpica/nspredef.c b/drivers/acpi/acpica/nspredef.c
index 3dbbecf..9d14b50 100644
--- a/drivers/acpi/acpica/nspredef.c
+++ b/drivers/acpi/acpica/nspredef.c
@@ -323,7 +323,7 @@
 
 	/*
 	 * Check the reference object for the correct reference type (opcode).
-	 * The only type of reference that can be converted to an union acpi_object is
+	 * The only type of reference that can be converted to a union acpi_object is
 	 * a reference to a named object (reference class: NAME)
 	 */
 	if (return_object->reference.class == ACPI_REFCLASS_NAME) {