fix: adjust bare-metal install guide

[phil-davis]

After make -C opencloud build the built artifacts are in a subdirectory called opencloud
In the opencloud subdirectory is the bin directory.

You can either:

• navigate down into the just-built opencloud directory, then follow the existing instructions after that, or;
• prepend /owncloud to each of the existing documented commands (e.g. ./opencloud/bin/opencloud server )

This PR just adds a cd opencloud to the instructions.