Update check_mysql_replication.sh

Hello,

I used that script to supervised my replication process.
I found myself in a "stupid" situation where slave data was modified instead of master -> replication Broke AND not detected.

So I had some Checking, and modified the positions calculus : 
Slave_1_position is now set to "Exec_Master_Log_Pos" and not "Read_Master_Log_Pos".

if slave is NOT in state : Slave_IO_Running=YES -> critical
if slave is NOT in state : Slave_SQL_Running=YES -> critical
if slave is NOT in state : Last_Errno=0 -> critical

I hope I made myself clear, english is not my birth language, and it's the first time I post something on Github.