Fix pin branch in installation instructions

It is not enough to pin Dune to the ocaml-wasm fork, one must also specify the right branch.

Co-authored-by: Jérôme Vouillon <jerome.vouillon@gmail.com>

README.md

@@ -26,7 +26,7 @@ utilizing
 
 The following commands will perform a minimal installation:
 ```
-opam pin add dune.3.13 https://github.com/ocaml-wasm/dune.git
+opam pin add dune.3.13 https://github.com/ocaml-wasm/dune.git#wasm
 opam pin add -n --with-version 5.3.0-wasm .
 opam install wasm_of_ocaml-compiler
 ```