fixed _update function example
fixed _update function example

It was only checking for req.id existance (it should
always be included in the request) and not its truthiness.

robertkowalski/couchdb-documentation,andywenk/couchdb-documentation,kxepal/couchdb-documentation,KimStebel/couchdb-documentation,kxepal/couchdb-documentation,micah/couchdb-documentation,robertkowalski/couch-doc-test,micah/couchdb-documentation,robertkowalski/couch-doc-test,KimStebel/couchdb-documentation,robertkowalski/couchdb-documentation,andywenk/couchdb-documentation,micah/couchdb-documentation,andywenk/couchdb-documentation,KimStebel/couchdb-documentation,robertkowalski/couch-doc-test,cloudant/couchdb-documentation,robertkowalski/couchdb-documentation,kxepal/couchdb-documentation,cloudant/couchdb-documentation