[bugfix][release] Fix duneproject's option for the enable all feature

A typo made it impossible to say 'no'.

Furthermore, I changed the text shown to the user to point to the
module documentation.

bin/duneproject

@@ -225,8 +225,8 @@ while [ "$DATACORRECT" != "y" -a "$DATACORRECT" != "Y" ]; do
   while [ -z "$MAINTAINER" ]; do
     read -p "4) Maintainer's email address? " MAINTAINER
   done
-  while [ "$ENABLE_ALL" != "y" -a "$ENABLE_ALL" != "Y" ]; do
-    read -p "5) Enable all found packages? (WE strongly recommend yes, unless you know exactly what you are doing) [y/N]" ENABLE_ALL
+  while [ "$ENABLE_ALL" != "y" -a "$ENABLE_ALL" != "N" ]; do
+    read -p "5) Enable all found packages? (This will result in a simplified build system. Read the doc in dune-common/cmake/modules/DuneEnableAllPackages.cmake for details.) [y/N]" ENABLE_ALL
   done
 
   echo