doc: describe argument of Symbol.for

Symbol is described with argument, but argument of Symbol.for
is ommited. So describe argument of Symbol.for for consistency.

Refs: https://github.com/nodejs/node/blob/main/doc/contributing/using-symbols.md#symbolstring
PR-URL: #46019
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>

doc/contributing/using-symbols.md

@@ -50,7 +50,7 @@ Some extremely popular modules in the ecosystem monkey patch some
 internals, making it impossible for us to update and improve those
 areas without causing issues for a significant amount of users.
 
-## `Symbol.for`
+## `Symbol.for(string)`
 
 Symbols created with `Symbol.for(string)` are global and unique to the
 same V8 Isolate. On the first call to `Symbol.for(string)` a symbol is