Fixed comments that do not reflect the PostGenerationContext changes.

[Antoliny0919]

In commit e19142c, the PostGenerationContext class was moved from builders.py to declarations.py with minor modifications.

class PostGenerationDeclaration(BaseDeclaration):
    ...
    def call(self, instance, step, context):  # pragma: no cover
        """Call this hook; no return value is expected.

        Args:
            instance (object): the newly generated object
            step (bool): whether the object was 'built' or 'created'
            context: a builder.PostGenerationContext containing values
                extracted from the containing factory's declaration
        """
        raise NotImplementedError()

Following the application of this commit, the call method's comments in the PostGenerationDeclaration class regarding the context parameter should update the reference from builder.PostGenerationContext to declarations.PostGenerationContext.