Yes that is the rule a runner must stay within 3 feet of the baseline per Rule 7.08:

> Any runner is out when—  (a)     (1)  He runs more than three
> feet away from his baseline to avoid being tagged unless his action is
> to avoid interference with a fielder fielding a batted ball.  A
> runner’s baseline is established when the tag attempt occurs and is a
> straight line from the runner to the base he is attempting to reach
> safely

Reference: http://www.mlb.com/mlb/downloads/y2007/07_the_runner.pdf