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: #17724
Fixes: #17694
Refs: #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>

tools/doc/html.js

@@ -416,15 +416,15 @@ const BSD_ONLY_SYSCALLS = new Set(['lchmod']);
 // '<a href="http://man7.org/linux/man-pages/man2/open.2.html">open(2)</a>'
 function linkManPages(text) {
   return text.replace(
-    /\b([a-z.]+)\((\d)([a-z]?)\)/gm,
-    (match, name, number, optionalCharacter) => {
+    /(^|\s)([a-z.]+)\((\d)([a-z]?)\)/gm,
+    (match, beginning, name, number, optionalCharacter) => {
       // name consists of lowercase letters, number is a single digit
       const displayAs = `${name}(${number}${optionalCharacter})`;
       if (BSD_ONLY_SYSCALLS.has(name)) {
-        return ` <a href="https://www.freebsd.org/cgi/man.cgi?query=${name}` +
+        return `${beginning}<a href="https://www.freebsd.org/cgi/man.cgi?query=${name}` +
           `&sektion=${number}">${displayAs}</a>`;
       } else {
-        return ` <a href="http://man7.org/linux/man-pages/man${number}` +
+        return `${beginning}<a href="http://man7.org/linux/man-pages/man${number}` +
           `/${name}.${number}${optionalCharacter}.html">${displayAs}</a>`;
       }
     });