Updates to http_protocol_proposal_v1.md

[nizarq]

Found some small trivial errors reading the document.

[nizarq]

when you say Implementation MUST provide read access to Correlation-Context for logging systems and MUST support adding properties to Correlation-Context. that feels a little confusing, because throughout the document you say Correlation-Context is read-only, which implies properties cannot be added to the collection. I think what you really mean is existing properties in the collection should be treated as read only but new properties are allowed.

[lmolkova]

Thanks!

I think our guidance here is a bit different:

1. First service should define correlation context.
2. Next service(s) MUST propagate it further without modifying properties, and SHOULD NOT add other properties.

So the Correlation-Context is indeed read only for every service except the first one, but it is not a strict requirement.

Implementation does not really know which service is the first one so it must allow to add properties anyway.

I agree that I should be more clear about it in the spec. BTW, there is an open PR to move this protocol to .net core repo. I'll do your changes there and also improve description.
Once PR is merged, I'll change this doc to refer to the corefx repo.