doc: add missing quotes in default string values

PR-URL: https://github.com/nodejs/node/pull/19894
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/https.md

@@ -160,7 +160,7 @@ changes:
 -->
 - `options` {Object | string | URL} Accepts all `options` from
   [`http.request()`][], with some differences in default values:
-  - `protocol` **Default:** `https:`
+  - `protocol` **Default:** `'https:'`
   - `port` **Default:** `443`
   - `agent` **Default:** `https.globalAgent`
 - `callback` {Function}