Fix installation instructions for *nix (GH-5605)


Remove pkg_add -r python from FreeBSD installation section.
Moved to OpenBSD.
(cherry picked from commit 3384d38d51a2c3450e742175db5d6d638fa5d2eb)

Co-authored-by: Eitan Adler <grimreaper@users.noreply.github.com>
diff --git a/Doc/using/unix.rst b/Doc/using/unix.rst
index 2da26d8..7719baa 100644
--- a/Doc/using/unix.rst
+++ b/Doc/using/unix.rst
@@ -41,9 +41,11 @@
 
 * FreeBSD users, to add the package use::
 
-     pkg_add -r python
+     pkg install python3
 
-* OpenBSD users use::
+* OpenBSD users, to add the package use::
+
+     pkg_add -r python
 
      pkg_add ftp://ftp.openbsd.org/pub/OpenBSD/4.2/packages/<insert your architecture here>/python-<version>.tgz