Fix position of Metasploit::Credential::Importer::Core docs
Fix position of Metasploit::Credential::Importer::Core docs

MSP-10130

Class comment was at beginning of file instead of immediately before the
`class` line, so the docs weren't being associated correctly.

rapid7/metasploit-credential,rapid7/metasploit-credential,rapid7/metasploit-credential