[Python] Fix function name for Pack method for structs

[kojung]

This PR fixes #8422 .

Before we get this:

class ABCWordT(object):
    # ABCWordT
    def Pack(self, builder):
        return CreateAbcword(builder, self.w)

Where the function CreateAbcword does not exist.

After we get this:

class ABCWordT(object):
    # ABCWordT
    def Pack(self, builder):
        return CreateABCWord(builder, self.w)

And the function CreateABCWord does exist.

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