add missing trailing slash

Author: eddelbuettel

docker/ci/Dockerfile

@@ -8,7 +8,7 @@ LABEL org.label-schema.license="GPL-2.0" \
 
 RUN apt-get update \
         && apt-get install -y --no-install-recommends \
-                libcurl4-openssl-dev
+                libcurl4-openssl-dev \
                 libssl-dev \
                 r-cran-inline \
                 r-cran-pkgkitten \