tools: fix man pages linking regex
The change to word boundary was breaking many doc pages. This reverts the word boundary back to space.
Fixes: https://github.com/nodejs/node/issues/17637 Fixes: https://github.com/nodejs/node/issues/17694 Refs: https://github.com/nodejs/node/pull/17479
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commit message follows commit guidelines
Affected core subsystem(s)
tools, doc