Help: add_dependencies requires at least two arguments

Update the documentation to match the implementation.

Help/command/add_dependencies.rst

@@ -5,13 +5,14 @@ Add a dependency between top-level targets.
 
 .. code-block:: cmake
 
-  add_dependencies(<target> [<target-dependency>]...)
+  add_dependencies(<target> <target-dependency>...)
 
 Makes a top-level ``<target>`` depend on other top-level targets to
 ensure that they build before ``<target>`` does.  A top-level target
 is one created by one of the :command:`add_executable`,
 :command:`add_library`, or :command:`add_custom_target` commands
 (but not targets generated by CMake like ``install``).
+At least one ``<target-dependency>`` must be given.
 
 Dependencies added to an :ref:`imported target <Imported Targets>`
 or an :ref:`interface library <Interface Libraries>` are followed