Fix const-prop/const-fold cross references

[davidmalcolm]

The "Constant Propagation" and "Constant Folding" sections each contain links to themselves; presumably they're meant to link to each other? Fixed in the following, I believe (though I messed up the whitespace slightly and github won't let me edit this anymore)

[vstinner]

No, they are links to the Optimization page, which is a different page. Optimizations in the FAT Python page are have a link starting with "fat-".

http://faster-cpython.readthedocs.org/fat_python.html#constant-propagation