Fix do not assign a lambda expression check.

def fix_e731(self, result):
        """Fix do not assign a lambda expression check."""
        (line_index, _, target) = get_index_offset_contents(result,
                                                            self.source)
        match = LAMBDA_REGEX.search(target)
        if match:
            end = match.end()
            self.source[line_index] = '{}def {}({}): return {}'.format(
                target[:match.start(0)], match.group(1), match.group(2),
                target[end:].lstrip())