docs: use consistent .append() in dis.rst

[nedbat]

The STACK variable is described as like a Python list, so pushing to it should be done with .append() consistently throughout.

As discussed at https://discuss.python.org/t/dis-py-stack-append-vs-stack-push/45330

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📚 Documentation preview 📚: https://cpython-previews--115434.org.readthedocs.build/

[terryjreedy]

The only instance of .push in 3.12 is the one before The operand determines which intrinsic function is called:. None in 3.11 doc. Not worth the backport hassle to me, but I would merge a backport if submitted and say how to do it if asked.