Update "Format String Syntax" documentation block to match actual behaviour

[pyctrl]

Hello, let me try to help a bit with docs.

Here is an string formatting example:

In [29]: "{a-b} {-}".format(**{"-": "_-_", "a-b": "ABC"})
Out[29]: 'ABC _-_'

This behaviour differs from Format String Syntax:

arg_name: [identifier | digit+]
attribute_name: identifier

So I updated arg_name and attribute_name syntax in that section to follow the actual behaviour:

• replaced identifier with attribute_name in arg_name
• set syntax for attribute_name as anything that doesn’t contain "." or "]"

You can find original discussion thread here

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[python-cla-bot]

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[StanFromIreland]

See: #127833

We could update the whole snippet now. I'd ask Petr for what his plan was but he's unavailable at the moment.