Fixed spelling + clarity (#4213)

Message given when password was too short was incorrectly spelled and was not clear enough.

options/locale/locale_en-US.ini

@@ -213,7 +213,7 @@ send_reset_mail = Click here to resend your password reset email
 reset_password = Reset Your Password
 invalid_code = Your confirmation code is invalid or has expired.
 reset_password_helper = Click here to reset your password
-password_too_short = Password length cannot be less then %d.
+password_too_short = Password length cannot be less than %d characters.
 non_local_account = Non-local users can not update their password through the Gitea web interface.
 verify = Verify
 scratch_code = Scratch code