Correct minor error in image documentation, and update logo (#1671)

* small correction noticed in rocker-org/rocker issue #381

url https://github.com/rocker-org/rocker/issues/381

* update R logo

official source under CC-BY-SA 4.0: https://www.r-project.org/logo/

r-base/content.md

@@ -33,7 +33,7 @@ $ docker run -ti --rm -v "$PWD":/home/docker -w /home/docker -u docker %%IMAGE%%
 Alternatively, just run a bash session on the container first. This allows a user to run batch commands and also edit and run scripts:
 
 ```console
-$ docker run -ti --rm %%IMAGE%% /usr/bin/bash
+$ docker run -ti --rm %%IMAGE%% bash
 $ vim.tiny myscript.R
 ```