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>

drivers/acpi/acpica/nsxfeval.c

@@ -495,9 +495,9 @@ static void acpi_ns_resolve_references(struct acpi_evaluate_info *info)
 	/*
 	 * Two types of references are supported - those created by Index and
 	 * ref_of operators. A name reference (AML_NAMEPATH_OP) can be converted
-	 * to an union acpi_object, so it is not dereferenced here. A ddb_handle
+	 * to a union acpi_object, so it is not dereferenced here. A ddb_handle
 	 * (AML_LOAD_OP) cannot be dereferenced, nor can it be converted to
-	 * an union acpi_object.
+	 * a union acpi_object.
 	 */
 	switch (info->return_object->reference.class) {
 	case ACPI_REFCLASS_INDEX: