Revert "Update README.md installation instructions."

This reverts commit a403ca3b67.

There are problems when running TPM with a `-b` flag, see #151.

README.md

@@ -30,7 +30,7 @@ set -g @plugin 'tmux-plugins/tmux-sensible'
 # set -g @plugin 'git@bitbucket.com:user/plugin'
 
 # Initialize TMUX plugin manager (keep this line at the very bottom of tmux.conf)
-run -b '~/.tmux/plugins/tpm/tpm'
+run '~/.tmux/plugins/tpm/tpm'
 ```
 
 Reload TMUX environment so TPM is sourced: