doc: fix broken using link in ffi.md

The `#sec-using-declarations` fragment does not exist in the explicit
resource management proposal, so the link silently resolves to the top
of the page. The proposal defines `using` under
`#sec-let-and-const-declarations`, and the only `using`-related section
ids are static semantics entries.

Point the reference at the MDN page for `using` instead, matching the
`[`using`]` definition already used in fs.md.

Signed-off-by: Soul Lee <alus20x@gmail.com>

Author: soulee-dev

doc/api/ffi.md

@@ -800,5 +800,5 @@ and keep callback and pointer lifetimes explicit on the native side.
 [`--allow-ffi`]: cli.md#--allow-ffi
 [`ffi.toBuffer(pointer, length, copy)`]: #ffitobufferpointer-length-copy
 [`library.functions`]: #libraryfunctions
-[`using`]: https://tc39.es/proposal-explicit-resource-management/#sec-using-declarations
+[`using`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/using
 [type names]: #type-names