Change the order so that alias of trail works properly. (#92)

Add missing `-p` to `mkdir`. Otherwise a pre-existing `traildepot` will lead to non-zero exit code.

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Co-authored-by: Sebastian Jeltsch <ignatz@users.noreply.github.com>

README.md

@@ -104,7 +104,7 @@ Alternatively, run TrailBase using the Docker image:
 
 ```sh
 alias trail='
-  mkdir traildepot && \
+  mkdir -p traildepot && \
   docker run \
       --network host \
       --mount type=bind,source="$PWD"/traildepot,target=/app/traildepot \