Naming improvement

`str` is used in the example too.

Author: mrmowji

1-js/05-data-types/05-array-methods/article.md

@@ -477,7 +477,7 @@ alert( str.split('') ); // t,e,s,t
 ```
 ````
 
-The call [arr.join(str)](mdn:js/Array/join) does the reverse to `split`. It creates a string of `arr` items glued by `str` between them.
+The call [arr.join(separator)](mdn:js/Array/join) does the reverse to `split`. It creates a string of `arr` items glued by `separator` between them.
 
 For instance: