Editorial: Remove the mention of the "the first substring"

[FrankYFTang]

Close #559
Remove the misleading usage of the "first" substring which imply it is possible to have second or rest but under condition which are not possible.

[ryzokuken]

@gibson042 @leobalter can either of you give this a final +1?

[anba]

Nit: There should be a "the" before each occurrence of "Unicode locale extension sequence" in the modified lines.

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The spec also contains steps like:

Let noExtensionsLocale be the String value that is locale with all Unicode locale extension sequences removed.

which also kind of implies that multiple Unicode locale extension sequences are possible within a language tag, even though a structurally valid language tag can contain at most one Unicode locale extension sequence. Should this also be changed, for example by switching from "all" to "any"?

Let noExtensionsLocale be the String value that is locale with any Unicode locale extension sequence removed.

[FrankYFTang]

@anba @ryzokuken PTAL

[ryzokuken]

LGTM, @anba @gibson042 @leobalter please approve.

[ryzokuken]

Merging editorial PR after multiple positive reviews and two editor signoffs.