docs: Fix a few typos

[timgates42]

There are small typos in:

• README.rst
• docs/why.rst
• src/switch_arm_gcc.h
• src/switch_ppc64_gcc.h
• tests/test_fibers.py

Fixes:

• Should read purpose rather than purporse.
• Should read popping rather than poping.
• Should read functionality rather than funcionality.
• Should read amount rather than ammount.
• Should read explicitly rather than explicitely.
• Should read above rather than avove.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md