Update functions.rst

[husisy]

For New-Style functions, one should use ".apply()" method instead of "call()".
I follow the documentation to run the origin code but fail with a RuntimeError. After changing to ".apply()", the code will pass.
(my second time to make a pull request, please tell if anything unsuitable. sincerely)

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[kmaehashi]

Thanks, LGTM!

[kmaehashi]

pfnCI, test this please.

[chainer-ci]

Jenkins CI test (for commit 0ae73d0, target branch master) succeeded!