doc: fix typo in assert code example

PR-URL: #9704
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/assert.md

@@ -339,7 +339,7 @@ assert.notStrictEqual(1, 2);
   // OK
 
 assert.notStrictEqual(1, 1);
-  // AssertionError: 1 != 1
+  // AssertionError: 1 !== 1
 
 assert.notStrictEqual(1, '1');
   // OK