Fix \u{X…XXXXXX} description: a hexadecimal value, not "1 to 6 bytes"

[1t1sCooL]

The \u{X…XXXXXX} escape takes hexadecimal digits, not bytes: the maximum code point 10FFFF is 6 hex digits, which is 3 bytes.

Following the review feedback, the digit-count limit is now dropped as well. The spec does not cap the number of hex digits inside \u{…} — it only requires the resulting value to be at most 0x10FFFF. For instance "\u{0000001}" has seven digits and is perfectly valid, so "1 to 6 digits" would have been inaccurate in its own way. The value range alone conveys everything that matters.

Spotted via the Russian translation (javascript-tutorial/ru.javascript.info#2124), where the same wording was faithfully translated along with the mistake. The Russian side is updated in javascript-tutorial/ru.javascript.info#2175.

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[1t1sCooL]

Good catch, thanks — fixed in 17ba8ad.

The spec doesn't cap the number of hex digits inside \u{…}; it only requires the resulting value to be at most 0x10FFFF. So "\u{0000001}" is valid and yields code point 1, which the "1 to 6" wording wrongly rejected.

I dropped the digit-count clause altogether rather than saying "at most six significant digits" — the value range already carries all the information that matters:

X…XXXXXX must be a hexadecimal value between 0 and 10FFFF (the highest code point defined by Unicode).

Note the original text was inaccurate on a second count as well: it said "1 to 6 bytes", where the escape actually takes hex digits. That was the reason for this PR in the first place.

The same sentence in the Russian translation is updated in javascript-tutorial/ru.javascript.info#2175 to keep the two in sync.