[FLINK-11584][docs] Add space in self-closing linebreak tag

[zentol]

What is the purpose of the change

2 issues are being addressed here:

• The configuration docs completeness test was parsing the document using html syntax, which has no support for self-closing tags. As a result the comparison of descriptions that contained linebreaks always failed, since we expected a self-closing tag (<br/>) but were returned something else (<br>).
• Jsoup (which we use for parsing html) always adds a space in self-closing tags, (i.e., <br /> instead of <br/>). Functionally these behave the same, and since the rest of our documentation also includes a space I have modified the HtmlFormatter accordingly.

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[NicoK]

looks good, except for the commit message which is misleading, maybe it should be two commits, one hotfix and then fix fix for FLINK-11584