Update menhir version in readme (#184)

Author: liam923

CONTRIBUTING.md

@@ -56,9 +56,9 @@ result accordingly.
 Merlin promotes the generated Menhir parser in its sources. This is done to
 avoid depending on Menhir when installing Merlin. However this also means that
 unnecessary diff will appear when the parser gets re-generated by a different
-version of Menhir. To remove this diff please use version `20201216`:
+version of Menhir. To remove this diff please use version `20231231`:
 ```bash
-$ opam pin menhir 20201216
+$ opam pin menhir 20231231
 ```
 The generated parser file should only be commited if there is an actual change
 in the grammar.

README.md

@@ -55,10 +55,10 @@ dune build -p dot-merlin-reader,merlin
 
 Note: if you want to work on Merlin, you'll want to avoid the `-p merlin` to
 build in dev mode, with some extra warnings enabled. In that case, you'll also
-need an extra dependency: Menhir. We recommend that you pin it to version 20201216
+need an extra dependency: Menhir. We recommend that you pin it to version 20231231
 which was used to generate the parser currently present in the sources.
 ```shell
-opam pin menhir 20201216
+opam pin menhir 20231231
 ```
 
 Installation