miniterm: fix double use of CTRL-T + s

use z for suspend instead

fixes #497
diff --git a/serial/tools/miniterm.py b/serial/tools/miniterm.py
index 2ec155e..13d0cdd 100644
--- a/serial/tools/miniterm.py
+++ b/serial/tools/miniterm.py
@@ -589,7 +589,7 @@
         #~ elif c == '\x0c':                       # CTRL+L -> cycle linefeed mode
         elif c in 'pP':                         # P -> change port
             self.change_port()
-        elif c in 'sS':                         # S -> suspend / open port temporarily
+        elif c in 'zZ':                         # S -> suspend / open port temporarily
             self.suspend_port()
         elif c in 'bB':                         # B -> change baudrate
             self.change_baudrate()