kitty: fixing environment variable substituition in docs/Makefile

The previous substitutution only worked by accident as make requires
environment variables to be enclosed in curly brackets as shown in this
excerpt from the build output:
> PYTHONPATH=YTHONPATH:.. HOME=MPDIR/nowhere sphinx-build -M html "." "_build" -T

The substituition is moved to the existing patch file to simplify the
builder expression.