doc: beautify net.md formats

PR-URL: #14987
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/net.md

@@ -50,7 +50,7 @@ added: v0.1.90
 
 This class is used to create a TCP or [IPC][] server.
 
-## new net.Server([options][, connectionListener])
+### new net.Server([options][, connectionListener])
 
 * Returns: {net.Server}