Update plugin installation instructions

Author: Bruno Sutic

README.md

@@ -29,10 +29,7 @@ In copy mode:
 
 Add plugin to the list of TPM plugins in `.tmux.conf`:
 
-    set -g @tpm_plugins "          \
-      tmux-plugins/tpm             \
-      tmux-plugins/tmux-open       \
-    "
+    set -g @plugin 'tmux-plugins/tmux-open'
 
 Hit `prefix + I` to fetch the plugin and source it. You should now be able to
 use the plugin.