Fix container path for cache dir

The code block contains the paths that should be mapped,
`/var/cache/nginx` and `/var/run` for readonly cases.
However, the description immediately above the code block
also listed two paths. The first, `/var/cache`, would require
additional dirs to already exist within the dir. For example,
the application will fail to start if the host didn't create
an `nginx` dir prior to starting the container.

This change updates the path in the description so it matches
the path mapped in the code block and no dirs need to exist
before starting the container.

nginx/content.md

@@ -110,7 +110,7 @@ This behavior can be changed via the following environment variables:
 
 ## Running %%IMAGE%% in read-only mode
 
-To run %%IMAGE%% in read-only mode, you will need to mount a Docker volume to every location where %%IMAGE%% writes information. The default %%IMAGE%% configuration requires write access to `/var/cache` and `/var/run`. This can be easily accomplished by running %%IMAGE%% as follows:
+To run %%IMAGE%% in read-only mode, you will need to mount a Docker volume to every location where %%IMAGE%% writes information. The default %%IMAGE%% configuration requires write access to `/var/cache/nginx` and `/var/run`. This can be easily accomplished by running %%IMAGE%% as follows:
 
 ```console
 $ docker run -d -p 80:80 --read-only -v $(pwd)/nginx-cache:/var/cache/nginx -v $(pwd)/nginx-pid:/var/run nginx