suggest correct package name in pin command

Summary:
public
This is more robust against infer being pinned from a directory not named
"infer/".

Reviewed By: jberdine

Differential Revision: D2864987

fb-gh-sync-id: ec2b1dd

m4/ac_assert_ocaml_pkg.m4

@@ -40,7 +40,7 @@ AC_DEFUN([AC_ASSERT_OCAML_PKG],
 If you are using opam, please run
 
   opam update
-  opam pin add --yes --no-action .
+  opam pin add --yes --no-action infer .
   opam install --deps-only infer])
   ])
 ])