[docs] Fix inputRef date picker customization demo

[Chee7ah]

Fixes Issue #2027.

Applying the inputRef instead of ref to the <input /> component in the demo fixes the issue of the picker closing prematurely after a single date pick instead of waiting for both dates of the range to be picked.

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[cypress]

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Test summary

78 • 0 • 3 • 0

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Run details

Project	material-ui-pickers
Status	Passed
Commit	d727761
Started	Jul 23, 2020 10:08 AM
Ended	Jul 23, 2020 10:10 AM
Duration	01:30 💡
OS	Linux Debian - 10.0
Browser	Chrome 80

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[oliviertassinari]

If we are going to fix the demo, I wonder if we shouldn't move forward with this approach at the same time: https://material-ui.com/components/autocomplete/#customizations - #1768. Maybe it will avoid us to break the demo a second time.

[oliviertassinari]

What about we leverage the focused variable for the custom input demo? I think that it's an important UX element developers and designers need to be aware of.