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/include/xen/interface/grant_table.h b/include/xen/interface/grant_table.h
index 56806bc..7fb7112 100644
--- a/include/xen/interface/grant_table.h
+++ b/include/xen/interface/grant_table.h
@@ -181,7 +181,7 @@
 };
 
 /*
- * Version 2 of the grant entry structure, here is an union because three
+ * Version 2 of the grant entry structure, here is a union because three
  * different types are suppotted: full_page, sub_page and transitive.
  */
 union grant_entry_v2 {