qapi: Fix dangling references to docs/devel/qapi-code-gen.txt

Conversion of docs/devel/qapi-code-gen.txt to ReST left several
dangling references behind.  Fix them to point to
docs/devel/qapi-code-gen.rst.

Fixes: f7aa076dbdfc (docs: convert qapi-code-gen.txt to ReST)
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240120095327.666239-4-armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
7 files changed