Integrate mdbook-spec for the reference.

This updates the reference which is now using a new mdbook plugin. This
requires a little extra work than a normal book because the plugin uses
`rustdoc` to generate links to the standard library. It also ensures
that the submodule is available for *any* command that uses rustbook,
since it is now part of the rustbook workspace.

1 files changed, 0 insertions, 0 deletions

diff --git a/src/doc/reference b/src/doc/reference
-Subproject e2f0bdc4031866734661dcdb548184bde1450ba
+Subproject 2e191814f163ee1e77e2d6094eee4dd78a289c5