Update API Heading to Sign-Statement

[SteveLasker]

The current heading says: "Issue Statement", which would refer to a document the issuer is looking to create/issue. This is simply a heading change to reflect the service will Sign the Statement on their behalf, differentiating the statement as a payload.

The heading and even the API name are another discussion as the doc, as written, isn't clear if it simply returns a signed statement, the client must re-submit under /entries, or the signed-statement/issue would also register. Based on that discussion, we may want to also rename the API.

• It's clear why the API would return the signed statement, for the client to hold and attach to a receipt.
• It's not clear why a user would need to re-submit a signed statement, requiring multiple round trips (see Workflow and details for signed-statement/issue #10)

[JAG-UK]

Clearer