#2 API typo (?) 'substract -> 'subtract'

[David Holroyd]

'substract' looks like a typo (though it is a real
word; I wasn't familliar with it):

Substract \Sub*stract"\, v. t. [F. suostraire; L.
subtus below
(from sub under) + trahere to draw. See {Substract}.]
To subtract; to withdraw. [Obs.] --Barrow.

This patch changes 'substract' to 'subtract'.

[David Holroyd]

Change 'substract' to 'subtract'