DOC: Update computation.rst
A wider span means a smaller alpha, which means each x_t has less impact on the ewma.

The proposed change to the documentation

y_t = (1 - \alpha) y_{t-1} + \alpha x_t 

would break this test: http://stackoverflow.com/a/17954898/190597
 @unutbu are the docs wrong or the computation?
 I believe the documention is correct as it stands.
 @vt100 this change would make the code and docs inconsistent

can u confirm?
 Closing request. Issue solved by Pull Request #4499
