core: options.py: correct 'enable_multicore_debug' option name (#1451)
core: options.py: correct 'enable_multicore_debug' option name (#1451)

Fixed the incorrect 'enable_multicore' session option name in options.py to match
'enable_multicore_debug' used when it's read (e.g., in cortex_m.py).
pyocd/pyOCD,flit/pyOCD,pyocd/pyOCD,flit/pyOCD