requires original syntax of chain for koan 53

[SherylHohman]

Using a more recent chain syntax, `_.chain(collection)`, will cause errors.
Koan 53 requires the original syntax of : `_(collection).chain`.

The more recent chain syntax was added more than 4 years ago,
and causes confusion when otherwise correct code produces errors.
Adding a note to koan 53 should remove this source of confusion
for users familiar with the newer `_.chain(collection)` syntax.

reference notes:
javascript-koans uses underscore v 1.1.6 from  Apr 2011
newer chain syntax was added in v 1.2.3 on 30 Dec 2011 as fix #408
current underscore library is v 1.8.3