Update examples/deprecated/mixedFF_structure/README.md
Update examples/deprecated/mixedFF_structure/README.md

Co-authored-by: Matt Thompson <52c81c13c8101ded72badbe0ecc58b92b7d0c20e@protonmail.com>
openforcefield/openff-toolkit,open-forcefield-group/openforcefield,openforcefield/openff-toolkit,open-forcefield-group/openforcefield,open-forcefield-group/openforcefield