CertPathSpi: fix encoding arg name

It appears an automated change has made "encoding" to "charsetName"
which doesn't make sense for the CertPath API since it's really an
encoding of the certificates that's being referenced.

Goes along with changes to CertPath made in change
228d2e19565a39e01849edd2f2b4d03e19d1ee3d

Change-Id: I05ea4adffa2100d4dcf78f8739bff13d40effebf
1 file changed