Fix typo in collection.set example

The example for collection.set called an undefined function "update".
Merge pull request #2398 from tranver/master

Fix typo in collection.set example
Merge pull request #2397 from wyuenho/collection-fetch-doc-fix

Update Collection#fetch's doc to refer to Collection#set
Merge branch 'master' of git://github.com/documentcloud/backbone into error-callback-doc-fix