nbdev_readme fails with mysterious error if docstring contains "\"

`nbdev_readme` fails if I have a backslash in the docstring of any exported function, e.g.

```
#| export
def f():
"""
Example doctring $x \mapsto y$
"""
return None
```
The resulting error message is very long and does not indicate the problem (although it does point to the offending function).

首先，使用其 docstring 包含所示反斜杠表达式的导出函数，通过 nbdev_readme 重现该失败。将生成的错误追溯到有问题的函数，并确定预期结果是成功处理，还是一条指出反斜杠问题的简洁消息。

由索引模型根据 Issue 内容生成。