doc: modernize and fix code examples in https.md

* Replace `var` by `const`.
* Comment out ellipses.
* Update code example (provide relevant file path, add missing option).

PR-URL: https://github.com/nodejs/node/pull/12171
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/https.md

@@ -70,7 +70,8 @@ const https = require('https');
 const fs = require('fs');
 
 const options = {
-  pfx: fs.readFileSync('server.pfx')
+  pfx: fs.readFileSync('test/fixtures/test_cert.pfx'),
+  passphrase: 'sample'
 };
 
 https.createServer(options, (req, res) => {
@@ -167,14 +168,14 @@ Example:
 ```js
 const https = require('https');
 
-var options = {
+const options = {
   hostname: 'encrypted.google.com',
   port: 443,
   path: '/',
   method: 'GET'
 };
 
-var req = https.request(options, (res) => {
+const req = https.request(options, (res) => {
   console.log('statusCode:', res.statusCode);
   console.log('headers:', res.headers);
 
@@ -191,7 +192,7 @@ req.end();
 Example using options from [`tls.connect()`][]:
 
 ```js
-var options = {
+const options = {
   hostname: 'encrypted.google.com',
   port: 443,
   path: '/',
@@ -201,8 +202,8 @@ var options = {
 };
 options.agent = new https.Agent(options);
 
-var req = https.request(options, (res) => {
-  ...
+const req = https.request(options, (res) => {
+  // ...
 });
 ```
 
@@ -211,7 +212,7 @@ Alternatively, opt out of connection pooling by not using an `Agent`.
 Example:
 
 ```js
-var options = {
+const options = {
   hostname: 'encrypted.google.com',
   port: 443,
   path: '/',
@@ -221,8 +222,8 @@ var options = {
   agent: false
 };
 
-var req = https.request(options, (res) => {
-  ...
+const req = https.request(options, (res) => {
+  // ...
 });
 ```