doc: change error message to 'not defined'

PR-URL: nodejs#26857
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

doc/api/errors.md

@@ -35,7 +35,7 @@ are handled using the [`try…catch` construct][try-catch] provided by the
 JavaScript language.
 
 ```js
-// Throws with a ReferenceError because z is undefined
+// Throws with a ReferenceError because z is not defined.
 try {
   const m = 1;
   const n = m + z;