Fix issue with XML line breaks inside vertex labels

[jsyang]

The regex needed to match the <br> or <br/> wasn't completely correct as there is an attempt to turn <br> elements into their XHTML counter-parts <br/> later in the flowRenderer sequence. This fixes the regex so both will match.

You can test this yourself with this MMD file:

graph LR

ap[Admin Web App]
hs[Host Server]
pa{Platform<br>Administrator}

pa --- |manages accounts| ap
ap --- |syncs accounts| hs

Without fix:

With fix

[coveralls]

Pull Request Test Coverage Report for Build 747

• 1 of 2 (50.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.002%) to 54.254%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/diagrams/flowchart/flowRenderer.js	1	2	50.0%

Totals
Change from base Build 716:	-0.002%
Covered Lines:	2051
Relevant Lines:	3755

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💛 - Coveralls

[knsv]

Thanks for your efforts!