Fix installation instructions for *nix (GH-5605)

Remove pkg_add -r python from FreeBSD installation section.
Moved to OpenBSD.
(cherry picked from commit 3384d38)

Co-authored-by: Eitan Adler <[email protected]>

Author: miss-islington

Doc/using/unix.rst

@@ -41,9 +41,11 @@ On FreeBSD and OpenBSD
 
 * FreeBSD users, to add the package use::
 
-     pkg_add -r python
+     pkg install python3
+
+* OpenBSD users, to add the package use::
 
-* OpenBSD users use::
+     pkg_add -r python
 
      pkg_add ftp://ftp.openbsd.org/pub/OpenBSD/4.2/packages/<insert your architecture here>/python-<version>.tgz