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: https://github.com/nodejs/node/pull/21357


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