Editorial: fix ref for break

[MattiasBuelens]

Clarify that the term "break" refers to the Infra standard, rather than the CSS standard.

Apparently this term has recently become ambiguous? See this failed CI job:

LINE 3582:85: Multiple possible 'break' dfn refs.
Arbitrarily chose https://drafts.csswg.org/css-break-4/#break
To auto-select one of the following refs, insert one of these lines into a <pre class=link-defaults> block:
spec:css-break-4; type:dfn; text:break
spec:infra; type:dfn; text:break
[=break=]
 ✘  Did not generate, due to errors exceeding the allowed error level.
make: *** [Makefile:23: deploy] Error 2

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