doc: use Class Method label consistently

The Buffer doc has a helpful "Class Method" label before static methods
on the Buffer class. The only other class in the docs that I can find
with static methods is ECDH in crypto. Add the label to the one static
method on the ECDH class.

PR-URL: #21357
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

doc/api/crypto.md

@@ -667,7 +667,7 @@ assert.strictEqual(aliceSecret.toString('hex'), bobSecret.toString('hex'));
 // OK
 ```
 
-### ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])
+### Class Method: ECDH.convertKey(key, curve[, inputEncoding[, outputEncoding[, format]]])
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