stmhal: Update help text.

Remove reference to pyb.gc; add reference to pyb.millis.

There are lots of functions not listed when you run help(), but it would
be too much to list them all, so we list only some basic, useful ones.

Addresses issue #846.

stmhal/help.c

@@ -39,8 +39,8 @@ STATIC const char *help_text =
 "\n"
 "Quick overview of commands for the board:\n"
 "  pyb.info()    -- print some general information\n"
-"  pyb.gc()      -- run the garbage collector\n"
 "  pyb.delay(n)  -- wait for n milliseconds\n"
+"  pyb.millis()  -- get number of milliseconds since hard reset\n"
 "  pyb.Switch()  -- create a switch object\n"
 "                   Switch methods: (), callback(f)\n"
 "  pyb.LED(n)    -- create an LED object for LED n (n=1,2,3,4)\n"