doc: fix stylistic issues in api/net.md

* Change var to const in an example of server creation.
* Add missing semicolons.
* Use `console` syntax highlighting in `telnet` and `nc` invocation
  examples and add shell prompt symbols to be consistent with the rest
  of the documentation.

PR-URL: nodejs#11786
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/net.md

@@ -66,7 +66,7 @@ Returns an object with `port`, `family`, and `address` properties:
 Example:
 
 ```js
-var server = net.createServer((socket) => {
+const server = net.createServer((socket) => {
   socket.end('goodbye\n');
 }).on('error', (err) => {
   // handle errors here
@@ -211,7 +211,7 @@ double-backslashes, such as:
 
 ```js
 net.createServer().listen(
-    path.join('\\\\?\\pipe', process.cwd(), 'myctl'))
+    path.join('\\\\?\\pipe', process.cwd(), 'myctl'));
 ```
 
 The parameter `backlog` behaves the same as in
@@ -874,8 +874,8 @@ server.listen(8124, () => {
 
 Test this by using `telnet`:
 
-```sh
-telnet localhost 8124
+```console
+$ telnet localhost 8124
 ```
 
 To listen on the socket `/tmp/echo.sock` the third line from the last would
@@ -889,8 +889,8 @@ server.listen('/tmp/echo.sock', () => {
 
 Use `nc` to connect to a UNIX domain socket server:
 
-```js
-nc -U /tmp/echo.sock
+```console
+$ nc -U /tmp/echo.sock
 ```
 
 ## net.isIP(input)