minor documentation typo

[briznad]

under the Change Watchers section on Polymer Kernel documentation page, in the code example there is a missing comma, which would produce a syntax error:

Steps to Reproduce:

1. go to http://www.polymer-project.org/polymer.html#change-watchers
2. view line 5 of the code example

Actual:
best: ''

Expected:
best: '',

[dfreedm]

Not sure when this change happened, but looks good now :)