Fix pluralize function

Signed-off-by: Etienne Millon <me@emillon.org>

src/dune_file.ml

@@ -1169,9 +1169,9 @@ module Executables = struct
 
   let pluralize s ~multi =
     if multi then
-      s
-    else
       s ^ "s"
+    else
+      s
 
   let common =
     let%map buildable = Buildable.t

test/blackbox-tests/test-cases/no-name-field/run.t

@@ -19,7 +19,7 @@ executable(s) stanza works the same way
   File "dune", line 1, characters 0-36:
   (executables (public_names foo bar))
   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-  Error: name field may not be omitted before dune version 1.1
+  Error: names field may not be omitted before dune version 1.1
   [1]
 
 there's only a public name but it's invalid as a name

test/blackbox-tests/test-cases/several-packages/run.t

@@ -9,13 +9,15 @@ displayed. This can happen for:
   I have the choice between these ones:
   - pkg1 (because of pkg1.opam)
   - pkg2 (because of pkg2.opam)
-  You need to add a (package ...) field to this (executables) stanza.
+  You need to add a (package ...) field to this (executable) stanza.
   [1]
 
 - (documentation)
 
   $ dune build --root documentation
   File "dune", line 1, characters 0-15:
+  (documentation)
+  ^^^^^^^^^^^^^^^
   Error: I can't determine automatically which package this stanza is for.
   I have the choice between these ones:
   - pkg1 (because of pkg1.opam)