Fix website : installation method for FreeBSD is deprecated

[Rodrigo Osorio]

Hi,

Browsing the project website I notice that the installation method for 
FreeBSD you describe
is deprecated, the correct syntax is `pkg install password-store`.

To give you more context, portmaster is a tool from the pkg_add era 
(before 2014). It still
maintained by some enthusiasts, but it's not recommended because 
sometime it breaks packages.
On the other side pkg is the official package tool the project maintains.

Best regards,
-- rodrigo