Remove bug in grep pattern

The pid the `grep -v` is trying to eliminate is not at the end
of the line.

`all_tmux_processes` produces the following output (for example):

```
tmux                        32069 29270
```

Here the first number is the pid and the second -- the uid. So, since
the idea of `number_tmux_processes_except_current_server` is to eliminate
32069 from consideration, it should look for "<SPACE>PID<SPACE>".

scripts/helpers.sh

@@ -34,7 +34,7 @@ all_tmux_processes() {
 
 number_tmux_processes_except_current_server() {
 	all_tmux_processes |
-		\grep -v " $(current_tmux_server_pid)$" |
+		\grep -v " $(current_tmux_server_pid) " |
 		wc -l |
 		sed "s/ //g"
 }