More defcustom fixes in ERC (Bug#66520)

More defcustom fixes in ERC (Bug#66520)

* lisp/erc/erc-fill.el (erc-fill-variable-maximum-indentation): Change
:type to choice, to allow nil.
* lisp/erc/erc-goodies.el (erc-keep-place-indicator-style): Fix
copy-pasta.
* lisp/erc/erc-networks.el (erc-networks-alist): Don't advertise that
MATCHER can be a function since it doesn't look like that's ever been
the case, reaching as far back as erc-networks.el's introduction to
the old CVS repo, in 2006.  (Bug#66520)