doc: fix createDiffieHellman generator type

OpenSSL does not provide a straight-forward way to implement a
non-integer generator, so createDiffieHellman never supported anything
other than a number as the generator. (This only applies to the
signature where the first argument is the size of the prime, and
therefore a number.)

Refs: nodejs/node-v0.x-archive#7086
Refs: nodejs#21782

doc/api/crypto.md

@@ -1855,8 +1855,7 @@ added: v0.5.0
 -->
 
 * `primeLength` {number}
-* `generator` {number | string | Buffer | TypedArray | DataView} **Default:**
-  `2`
+* `generator` {number} **Default:** `2`
 * Returns: {DiffieHellman}
 
 Creates a `DiffieHellman` key exchange object and generates a prime of