small modification to fix bond/react to allow equal style variables as probability o

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Changes to be committed:
	modified:   doc/src/fix_bond_react.rst
	new file:   examples/USER/reaction/nylon,6-6_melt/in.large_nylon_melt_variable_probability
	modified:   src/USER-REACTION/fix_bond_react.cpp
	modified:   src/USER-REACTION/fix_bond_react.h