[docs] Quoting in Format Grammar

[Heinrich Hartmann]

Hi there,

in the str.format grammar you are very careful to get the quoting of
terminal symbols right, e.g. "!", ":" are all quoted.

However, in the format_spec grammar you are less careful with quoting
terminal symbols:

```
.. productionlist:: sf
   format_spec:
[[`fill`]`align`][`sign`][#][0][`width`][,][.`precision`][`type`]
```

IMHO symbols "#", "0", ",", "." should all be quoted.

Or am I missing something here?

Best regards,
Heinrich Hartmann


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Dr. Heinrich Hartmann
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