doc: fix typo in http.md

the returns need to be lowercase

PR-URL: #43933
Reviewed-By: Paolo Insogna <paolo@cowtech.it>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>

doc/api/http.md

@@ -866,7 +866,7 @@ request.setHeader('Foo', 'bar');
 request.setHeader('Cookie', ['foo=bar', 'bar=baz']);
 
 const headerNames = request.getHeaderNames();
-// headerNames === ['foo', 'Cookie']
+// headerNames === ['foo', 'cookie']
 ```
 
 ### `request.getHeaders()`