Add a missing “:” on line 145

See Issue https://github.com/jupyter/repo2docker/issues/898

docs/source/config_files.rst

@@ -142,7 +142,7 @@ We use ``apt.txt``, for example, to install LaTeX in our
 
 To install your repository like an R package, you may include a
 ``DESCRIPTION`` file. repo2docker installs the package and dependencies
-from the ``DESCRIPTION`` by running ``devtools:install_git(".")``.
+from the ``DESCRIPTION`` by running ``devtools::install_git(".")``.
 
 You also need to have a ``runtime.txt`` file that is formatted as
 ``r-<YYYY>-<MM>-<DD>``, where YYYY-MM-DD is a snapshot of MRAN that will be