doc: fix crypto spkac function descriptions

Fix regarding description of the following functions:

Certificate.exportPublicKey(spkac)
Certificate.exportChallenge(spkac)

The descriptions were applied incorrectly.

PR-URL: https://github.com/nodejs/node/pull/3614
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/crypto.markdown

@@ -673,11 +673,11 @@ Returns true of false based on the validity of the SPKAC.
 
 ### Certificate.exportChallenge(spkac)
 
-Exports the encoded public key from the supplied SPKAC.
+Exports the encoded challenge associated with the SPKAC.
 
 ### Certificate.exportPublicKey(spkac)
 
-Exports the encoded challenge associated with the SPKAC.
+Exports the encoded public key from the supplied SPKAC.
 
 ## crypto.publicEncrypt(public_key, buffer)