docs: update info command for newer PyVISA

In PyVISA’s repo, fcd3728 (updated commands to pyvisa-shell and
pyvisa-info, 2017-11-22) [1] replaced

	python -m visa

with the pyvisa-shell and pyvisa-info commands. At this point, you have
to use pyvisa-info. Running

	python -m visa info

will no longer work.

[1]: <pyvisa/pyvisa@fcd3728>

docs/source/installation.rst

@@ -74,7 +74,7 @@ How do I know if PyVISA-py is properly installed?
 
 Using the pyvisa information tool. Run in your console::
 
-  python -m visa info
+  pyvisa-info
 
 You will get info about PyVISA, the installed backends and their options.