stringformat docs should link directly to the formatting docs

[Dougal Sutherland]

The reference for the stringformat filter links to ​http://docs.python.org/lib/typesseq-strings.html, an outdated url which redirects to ​http://docs.python.org/library/stdtypes.html, a giant file describing all of python's standard types. It would make more sense to link directly to the string formatting docs.

[Russell Keith-Magee]

(In [9863]) Fixed #10007 -- Corrected (and narrowed) a reference to the Python standard library documentation. Thanks to d00gs for the report.