doc: fix typo in assert code example

PR-URL: https://github.com/nodejs/node/pull/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

@@ -345,7 +345,7 @@ assert.notStrictEqual(1, 2);
 // OK
 
 assert.notStrictEqual(1, 1);
-// AssertionError: 1 != 1
+// AssertionError: 1 !== 1
 
 assert.notStrictEqual(1, '1');
 // OK