[docs] Doc errors

[王越]

Hi, I find a mistake on https://docs.python.org/zh-cn/3.7/bugs.html, maybe
it is because the translation.
In the forth paragraph, "解决方案是使用 Python 的原始字符串表示法来表达正则样式；在以 'r' 为前缀的字符串文本中,
反斜杠没有任何转义。 因此,``r"n"`` 是一个包含 '\' 和 'n' 的双字符字符串, 而 "\n" 是包含换行符的一个字符字符串。
我们一般用原始表示法来表达样式。" while the original express is " So r"\n" is a
two-character string containing".
There is a missing "\" and should be "r"\n"".
Thank you!

Best Wishes,
Bruce Wang
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