(Chapter 7.14) Invalid command line interaction

[grigoriosgiann]

The issue lies with the source code, therefore it exists in all versions.

Describe the bug:
In #542 the git credential store extension was changed, however, the command line interaction using it was not updated accordingly.

Expected behavior:
The username should be passed as a parameter in the command line snippet.
Kinda like this:

$ git credential-read-only --file=/mnt/shared/creds get
protocol=https
host=mygithost
username=bob

protocol=https
host=mygithost
username=bob
password=s3cre7

Additional context:
Link to the relevant file: https://github.com/progit/progit2/blob/master/book/07-git-tools/sections/credentials.asc