Fix missing space before troubleshooting link in fatal error handler

[Presskopp]

Adds whitespace between fatal error message and troubleshooting link. Prevents sentence concatenation like "forums.Learn more".

Trac ticket: #64527

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[github-actions]

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[t-hamano]

Let't close this PR as adding whitespace between paragraphs isn't an ideal approach. See https://core.trac.wordpress.org/ticket/64527#comment:3