words: add word we forgot

I explained this tails/scales list to my 5yo and he looked at me like
it was the most obvious idea ever. Of course we'd make such lists at
work!  What else do grown-ups do all day? And then he wouldn't stop
talking about coelacanths and I had no clue what he was saying or how
to spell it until I asked my phone and the phone apparently understood
me and I realized it was a fish and he was helping me? I think?

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

words/scales.txt

@@ -318,3 +318,4 @@ uaru
 vimba
 wahoo
 zebra
+coelacanth

words/tails.txt

@@ -540,3 +540,4 @@ tailor
 uaru
 vimba
 wahoo
+coelacanth