OCaml.gitignore: Make a comment a bit more precise

Resources:

    - https://github.com/ocaml/ocamlbuild/blob/master/manual/manual.adoc#110-source-and-build-directories-module-paths-include-paths
    - https://dune.readthedocs.io/en/stable/usage.html#custom-build-directory

OCaml.gitignore

@@ -8,7 +8,7 @@
 *.cmxs
 *.cmxa
 
-# ocamlbuild working directory
+# ocamlbuild and Dune default working directory
 _build/
 
 # ocamlbuild targets