DOC: config: Fix a typo on ssl_c_chain_der

DOC: config: Fix a typo on ssl_c_chain_der

There is a typo on the ssl_c_chain_der sample fetch
(s/ssl_c_der_chain/ssl_c_chain_der/). This implies a move of the fetch to keep
it at the right place.

This should be backported as far as 2.2 or anywhere the commit a598b500b
("MINOR: ssl: add ssl_{c,s}_chain_der fetch methods") is.