Update readme to use 4.12

Signed-off-by: Rudi Grinberg <[email protected]>

Author: rgrinberg

README.md

@@ -100,7 +100,7 @@ git clone --recursive [email protected]:ocaml/ocaml-lsp.git
 git submodule update --init --recursive
 
 # create local switch (or use global one) and install dependencies
-opam switch create . ocaml-base-compiler.4.11.1 --with-test
+opam switch create . ocaml-base-compiler.4.12.0 --with-test
 
 # don't forget to set your environment to use the local switch
 eval ($opam env)