Fixing duplicate tag and updating for github migration. fixes <https://github.com/ravenbrook/mps/issues/155#issuecomment-1430979446>.

mps/design/doc.txt

@@ -93,11 +93,14 @@ _`.impl.design`: Design documents are written in plain RST (with no
 custom directives) to meet `.req.design.standalone`_.
 
 _`.impl.design.pelican`: Design documents are converted to HTML using
-|RstReader|_.
+|RstReader|_ as part of `Charlotte
+<https://info.ravenbrook.com/project/charlotte>`__.
 
 .. |RstReader| replace:: ``pelican.readers.RstReader``
 .. _RstReader: https://fossies.org/dox/pelican-3.7.1/classpelican_1_1readers_1_1RstReader.html
 
+_`.impl.design.github`: Design documents are also `rendered as HTML by GitHub <https://docs.github.com/en/repositories/working-with-files/using-files/working-with-non-code-files#rendering-differences-in-prose-documents>`__.
+
 _`.impl.manual`: The manual is written in RST using Sphinx_ extensions
 and custom manual extensions (see `.ext`_).
 
@@ -206,8 +209,8 @@ is translated into a ``c:function`` directive::
     .. c:function:: void LandFinish(Land land)
 
 
-_`.fmt.function-decl`: A paragraph consisting of a macro
-declaration on a single line formatted as code, for example::
+_`.fmt.macro-decl`: A paragraph consisting of a macro declaration on a
+single line formatted as code, for example::
 
     ``RING_FOR(node, ring, next)``
 
@@ -323,14 +326,16 @@ Document History
 ----------------
 
 - 2018-09-18 GDR_ Created based on [RB_2013-05-09]_.
+- 2023-02-15 RB_ Updating for migration to GitHub.
 
 .. _GDR: https://www.ravenbrook.com/consultants/gdr/
+.. _RB: mailto:rb@ravenbrook.com
 
 
 Copyright and License
 ---------------------
 
-Copyright © 2018–2020 `Ravenbrook Limited <https://www.ravenbrook.com/>`_.
+Copyright © 2018–2023 `Ravenbrook Limited <https://www.ravenbrook.com/>`_.
 
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions are