Changes to introduction of http signing section

Issue #625 resolved
Joseph Heenan created an issue

https://openid.bitbucket.io/fapi/fapi-2_0-message-signing.html#name-http-message-signing

currently says:

To support non-repudiation for NR5 and NR6, HTTP requests and responses can be signed.

A future version of this profile expects to support HTTP Message Signing using the _HTTP Message Signatures_specification being developed by the IETF HTTP Working Group.

I think the second paragraph should have been removed.

I suggest to change the first sentence to:

To support non-repudiation for NR5 and NR6, HTTP requests, or responses, or both can be signed.

to make it clearer that it is possible to only have one of them signed.