README: Use relative links to the proposals

There's no need to use absolute links for these.  And using relative
links sets a good example for future proposals, because the links will
work in the proposal branch's README before the referenced proposal
lands in the TOB master.

There's a similar absolute link for the code-of-conduct in the
distribution proposal.  The intention may have been to show the
absolute URL which would be included in new project's repository.  I
don't think that matters, because the project template has included a
similar link since opencontainers/project-template@ee72bc89
(CONTRIBUTING: Code of conduct, meetings, mailing list, and IRC,
2016-09-08, opencontainers#45), so I've left that alone here.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

README.md

@@ -20,11 +20,11 @@ https://groups.google.com/a/opencontainers.org/forum/#!forum/tob (tob@opencontai
 
 ## Project Proposals
 
-* [Digest](https://github.com/opencontainers/tob/blob/master/proposals/digest.md)
-* [Distribution Spec](https://github.com/opencontainers/tob/blob/master/proposals/distribution.md)
-* [Image Format Spec](https://github.com/opencontainers/tob/tree/master/proposals/image-format)
-* [SELinux](https://github.com/opencontainers/tob/blob/master/proposals/selinux.md)
-* [Tools](https://github.com/opencontainers/tob/blob/master/proposals/tools.md)
+* [Digest](proposals/digest.md)
+* [Distribution Spec](proposals/distribution.md)
+* [Image Format Spec](proposals/image-format)
+* [SELinux](proposals/selinux.md)
+* [Tools](proposals/tools.md)
 
 ## Voting