ssh/tailssh: fix typo in error message

"look up" is the verb. "lookup" is a noun.

Change-Id: I81c99e12c236488690758fb5c121e7e4e1622a36
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/tailssh.go

@@ -368,8 +368,8 @@ func (c *conn) doPolicyAuth(ctx ssh.Context, pubKey ssh.PublicKey) error {
 		}
 		lu, err := user.Lookup(localUser)
 		if err != nil {
-			c.logf("failed to lookup %v: %v", localUser, err)
-			ctx.SendAuthBanner(fmt.Sprintf("failed to lookup %v\r\n", localUser))
+			c.logf("failed to look up %v: %v", localUser, err)
+			ctx.SendAuthBanner(fmt.Sprintf("failed to look up %v\r\n", localUser))
 			return err
 		}
 		gids, err := lu.GroupIds()