Update package for checking CredentialMsgType

[NhoxxKienn]

Signed-off-by: Minh Huy, Tran [email protected]

Use the following template for your PR (make sure you follow our guidelines:

Title: Update outdated doc.go

Description: The package for checking CredentialMsgType is currently not presentproof but didcomm/protocol/issuecredential

Summary: Change presentproof to an appropriate package issuecredential and Match the type of the MsgType with the example_test.go (V2)