test: change misleading variable name

The return value of getPrime() is not a private key in any way.

Refs: nodejs/node-v0.x-archive#2638

PR-URL: nodejs#43990
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Feng Yu <F3n67u@outlook.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/pummel/test-crypto-dh-hash.js

@@ -50,10 +50,10 @@ const hashes = {
 
 for (const name in hashes) {
   const group = crypto.getDiffieHellman(name);
-  const private_key = group.getPrime('hex');
+  const prime = group.getPrime('hex');
   const hash1 = hashes[name];
   const hash2 = crypto.createHash('sha1')
-                    .update(private_key.toUpperCase()).digest('hex');
+                    .update(prime.toUpperCase()).digest('hex');
   assert.strictEqual(hash1, hash2);
   assert.strictEqual(group.getGenerator('hex'), '02');
 }