docs: use crypto.randomBytes in DiskStorage filename example

[luisangelrod]

Summary

The README's DiskStorage example uses Math.random() to generate unique filenames. This is a security antipattern that developers copy into production code, as documented in #1386.

Math.random() uses V8's xorshift128+ PRNG, which is not cryptographically secure — its state can be recovered from a small number of outputs. Combined with Date.now() (which an attacker can narrow to ~1 second), the generated filename provides only ~30 bits of unpredictable entropy. In web-accessible upload directories this is enough for an attacker to enumerate filenames and access other users' files.

Multer's own internal default already does this correctly — storage/disk.js uses crypto.randomBytes(16). This PR aligns the README example with that existing best practice.

Changes

• Require crypto (Node.js built-in, no new dependency)
• Replace Date.now() + Math.random() with crypto.randomBytes(16)
• Add brief comments explaining why Math.random() is inappropriate here

Closes #1386