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/scripts/spelling.txt b/scripts/spelling.txt
index 1379453..27991a9 100644
--- a/scripts/spelling.txt
+++ b/scripts/spelling.txt
@@ -86,6 +86,7 @@
 ambigious||ambiguous
 amoung||among
 amout||amount
+an union||a union
 an user||a user
 an userspace||a userspace
 analysator||analyzer