tools: remove redundant RegExp flag

PR-URL: #20309
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

tools/doc/preprocess.js

@@ -6,7 +6,7 @@ const path = require('path');
 const fs = require('fs');
 
 const includeExpr = /^@include\s+([\w-]+)(?:\.md)?$/gmi;
-const commentExpr = /^@\/\/.*$/gmi;
+const commentExpr = /^@\/\/.*$/gm;
 
 function processIncludes(inputFile, input, cb) {
   const includes = input.match(includeExpr);