doc: use DUNE_GEN instead of JBUILDER_GEN (#1725)

Both symbols are supported, but of course `DUNE_GEN` is the
preferred one in new code.

Signed-off-by: Anil Madhavapeddy <[email protected]>

Author: avsm

doc/advanced-topics.rst

@@ -19,15 +19,15 @@ In order to do that, write or setup a rule to generate a
 ``META.<package>.template`` file in the same directory as the
 ``<package>.opam`` file. Dune will generate a ``META.<package>``
 file from the ``META.<package>.template`` file by replacing lines of
-the form ``# JBUILDER_GEN`` by the contents of the ``META`` it would
+the form ``# DUNE_GEN`` by the contents of the ``META`` it would
 normally generate.
 
 For instance if you want to extend the ``META`` file generated by
 dune you can write the following ``META.foo.template`` file:
 
 .. code::
 
-   # JBUILDER_GEN
+   # DUNE_GEN
    blah = "..."
 
 Findlib integration and limitations