tools: fix man pages linking regex

The change to word boundary was breaking many doc pages. This reverts
the word boundary back to space.

PR-URL: https://github.com/nodejs/node/pull/17724
Fixes: https://github.com/nodejs/node/issues/17694
Refs: https://github.com/nodejs/node/pull/17479


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>