minor typo in deep copy

I came across this when compiling against copy.py, looking to use the deepcopy function

File "/lib/copy.py", line 194, in deepcopy
NameError: name 'dispatch_table' is not defined

I believe it should be _deepcopy_dispatch

https://github.com/micropython/micropython-lib/blame/e4cf09527bce7569f5db742cf6ae9db68d50c6a9/python-stdlib/copy/copy.py#L96