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

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

  overwrien||overwritten

Link: http://lkml.kernel.org/r/1481573103-11329-30-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/isdn/hardware/mISDN/mISDNipac.c

@@ -1625,7 +1625,7 @@ mISDNipac_init(struct ipac_hw *ipac, void *hw)
 		ipac->hscx[i].bch.hw = hw;
 		ipac->hscx[i].ip = ipac;
 		/* default values for IOM time slots
-		 * can be overwriten by card */
+		 * can be overwritten by card */
 		ipac->hscx[i].slot = (i == 0) ? 0x2f : 0x03;
 	}