[format.context] Fix incorrect example

[hewillk]

isdigit() is not a contexpr function so it cannot be used in parse(), and the alignment seems to be in the wrong direction.
Demo of the original example: https://godbolt.org/z/qbs6dvq9q
fixed: https://godbolt.org/z/sjWdxxzWc

[hewillk]

Sorry, I think there is still something wrong with this example, because if (value < 0 || value > numeric_limits<int>::max()) will trigger the warning of comparison of constant '0' with boolean expression is always false: https://godbolt.org/z/5h7TsfdPE
@jwakely, any thoughts?

[frederick-vs-ja]

Sorry, I think there is still something wrong with this example, because if (value < 0 || value > numeric_limits<int>::max()) will trigger the warning of comparison of constant '0' with boolean expression is always false: https://godbolt.org/z/5h7TsfdPE
@jwakely, any thoughts?

The current example is totally correct to me. Not sure whether it's a good idea to make examples in the standard warning-free.

[jwakely]

I agree, we don't need to fix warnings. The bool case works correctly despite the warnings (and isn't actually used at runtime anyway).