reduce max password length on costco.com (#758)

The rendered password input has `maxlength="16"`, and the "Create
Account" page lists "Use between 8 and 16 characters". Decrease the
maximum password length.

quirks/password-rules.json

@@ -204,7 +204,7 @@
         "password-rules": "minlength: 6; maxlength: 15; allowed: lower, upper, digit, [-.];"
     },
     "costco.com": {
-        "password-rules": "minlength: 8; maxlength: 20; required: lower, upper; allowed: digit, [-!#$%&'()*+/:;=?@[^_`{|}~]];"
+        "password-rules": "minlength: 8; maxlength: 16; required: lower, upper; allowed: digit, [-!#$%&'()*+/:;=?@[^_`{|}~]];"
     },
     "coursera.com": {
         "password-rules": "minlength: 8; maxlength: 72;"