[2.1] Inconsistent title vs type name: 6.6.2

[sr@ctin.us]

Fwd from: https://github.com/oasis-tcs/cti-stix2/issues/181

[2.1] Inconsistent title vs type name: 6.6.2

Section 6.6.2 the header reads "Email MIME Component Type" but the  
type name says "mime-part-type". Should the header be renamed to  
"Email MIME Part Type" ?

@rpiazza then posted:

Network Socket Extension is named socket-ext. If we think its worth  
changing mime-part-type, we should change this one too. I'm fine  
either way.