Update nondiff_points.md (#562)

typo

Author: matbesancon

docs/src/maths/nondiff_points.md

@@ -112,7 +112,7 @@ Here we have no real choice but to say the derivative at `0` is `Inf`.
 We could consider as an alternative saying some large but finite value.
 However, if too large it will just overflow rapidly anyway; and if too small it will not dominate over finite terms.
 It is not possible to find a given value that is always large enough.
-Our alternatives  woud be to consider the dederivative at `nextfloat(0.0)` or `prevfloat(0.0)`.
+Our alternatives  woud be to consider the derivative at `nextfloat(0.0)` or `prevfloat(0.0)`.
 But this is more or less the same as choosing some large value -- in this case an extremely large value that will rapidly overflow.