Update 2.4-upgrade.txt

Fixed text - referenced 2.0 when it should've said 2.2.

(Side question, maybe worth a DOCS ticket: should we have instructions for going from 2.0 -> 2.4?  Or just let users do the two-step from 2.0 -> 2.4?)

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