Formatting errors in INSTALL.rst

[s-mcf]

INSTALL.rst has a few errors which make the installation process somewhat confusing, and could result in the user installing an unusable installation of A2Billing. Specifically:

• Shell commands are formatted as bullets instead of literals, which makes copying and pasting difficult
• Some command blocks and configuration files are collapsed onto one line
• One command copied a dangling symlink into the AGI folder
• The current stable version of Asterisk uses a different argument separator for AGI calls than assumed in the given extensions.conf configuration

I've submitted a pull request which should fix these errors.

[areski]

Thanks you dont need to open an issue if you open a PR