fix whitespace

X11/Dockerfile

 FROM rocker/r-ver:latest 
 
 ## R's X11 runtime dependencies
-RUN apt-get update \ 
+RUN apt-get update \
   && apt-get install -y --no-install-recommends \
     libx11-6 \
     libxss1 \
-    libxt6 \ 
+    libxt6 \
     libxext6 \
     libsm6 \
     libice6 \
-    xdg-utils \ 
+    xdg-utils \
   && rm -rf /var/lib/apt/lists/*