DOCS-1852: update reference-operators page

Signed-off-by: Sam Kleinman <samk@10gen.com>
Update 2.6.txt

Quotes was required for it to work on my 2.5.2.

Signed-off-by: Sam Kleinman <samk@10gen.com>
updated link

seems like the original 'Walkthrough MongoDB Data Modeling' link is dead?  Googled and found this.

Signed-off-by: Sam Kleinman <samk@10gen.com>