Restore old MultiMarkdown.pl formatting to spec

The API service uses Text::MultiMarkdown to format the spec, and that
module hasa not been updated to support fenced code blocks or curly
quotes. So revert completely to the spec as released in v0.32.0, only
changing inline links to reference links.

Changes

@@ -1,8 +1,8 @@
 Revision history for Perl extension PGXN::Manager
 
 0.32.1
-       - Updated `doc/spec.txt` to v1.0.1, from it new home in
-         https://github.com/pgxn/pgxn-meta-spec.
+       - Converged inline links in `doc/spec.txt` to reference links for
+         slightly better legibility as plain text.
        - Fixed import quoted version warning with Perl 5.40.
        - Fixed validation of rsync URLs in the mirror editor.
        - Added Content Security Policy notes to the README to ensure that