docs(contributing): fix jest reference in vitest mock example (#3996)

The "Now (vitest)" example under "Mocking modules" uses `jest.fn()`
instead of `vitest.fn()` when mocking `os.freemem`. This is leftover
from the Jest to vitest migration.

`vitest.config.mts` sets `test.globals: true` with no jest alias, so
copying this example as written would throw "ReferenceError: jest is not
defined" in a real test file. Every other example in the same guide
correctly uses `vitest.fn()`.

CONTRIBUTING.md

@@ -110,7 +110,7 @@ vitest.mock('node:os', async (importActual) => {
     return {
         ...original,
         platform: () => 'darwin',
-        freemem: jest.fn(),
+        freemem: vitest.fn(),
     };
 });
 ```