Use “Ten Frames” (plural) for the accordion box title in “Twenty” screen

[pixelzoom]

From https://github.com/phetsims/number-suite-common/issues/3:

• [ ] Should we use “Ten Frames” (plural) for the accordion box title in “Twenty” screen? (effort: trivial)

@amanda-phet replied:

That sounds fine.

I'll knock this one off.

[pixelzoom]

Transferring this issue to number-play, since it's relevant only for the Twenty screen in Number Play.

[pixelzoom]

Done in the above commits.

Ten screen:

Twenty screen:

Over to @chrisklus to review. Feel free to close.

[chrisklus]

Looks great, thanks!