doc: use ffi.suffix in permission example

The `--allow-ffi` example in cli.md hard-coded the Linux-only `.so`
extension. node:ffi already exposes `suffix` for exactly this case,
and doc/api/ffi.md's own examples use it. Do the same here.

Assisted-by: Claude Sonnet 5
Signed-off-by: agape1225 <49804691+agape1225@users.noreply.github.com>
PR-URL: #64914
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

Author: agape1225

doc/api/cli.md

@@ -208,8 +208,8 @@ starting Node.js. The [`node:ffi`][] module also requires the
 Example:
 
 ```js
-const { DynamicLibrary } = require('node:ffi');
-const lib = new DynamicLibrary('mylib.so');
+const { DynamicLibrary, suffix } = require('node:ffi');
+const lib = new DynamicLibrary(`./mylib.${suffix}`);
 ```
 
 ```console

doc/node.1

@@ -126,8 +126,8 @@ starting Node.js. The \fBnode:ffi\fR module also requires the
 \fB--experimental-ffi\fR flag and is only available in builds with FFI support.
 Example:
 .Bd -literal
-const { DynamicLibrary } = require('node:ffi');
-const lib = new DynamicLibrary('mylib.so');
+const { DynamicLibrary, suffix } = require('node:ffi');
+const lib = new DynamicLibrary(`./mylib.${suffix}`);
 .Ed
 .Bd -literal
 $ node --permission --experimental-ffi index.js