Update README.md installation instructions.

`run '~/.tmux/plugins/tpm/tpm'` should be run in the background, so the `-b` flag was added.

Author: metakirby5

README.md

@@ -29,7 +29,7 @@ set -g @plugin 'tmux-plugins/tmux-sensible'
 # set -g @plugin '[email protected]/user/plugin'
 
 # Initialize TMUX plugin manager (keep this line at the very bottom of tmux.conf)
-run '~/.tmux/plugins/tpm/tpm'
+run -b '~/.tmux/plugins/tpm/tpm'
 ```
 
 Reload TMUX environment so TPM is sourced: