DOC: Fixing formula in docstrings

The formula did not match the implementation.

Also, x and t were mixed as independent variable. Changed to x, following the example of others.
Merge pull request #2966 from Dapid/wavelets

DOC: Fixing formula in docstrings
updated ss2tf docstring in response to Issue #2398
DOC: clean up signal.ss2tf docstring a bit more.  Closes gh-2398.
Merge branch 'pr/2959' into master.

See https://github.com/scipy/scipy/pull/2959