Minor fix docs to prevent deprecation warning
Minor fix docs to prevent deprecation warning

Running tests from securedrop/securedrop prints a deprecation warning.

The bin/dev-shell script sets TOPLEVEL from Git output and
doesn't depend on the current directory, so there is no need to
change directory before running the bin/dev-shell examples.

The lines become longer, but still fit the Read the Docs code block
width in all orientations of an iPad-sized device and readability
remains unchanged.

conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop,conorsch/securedrop