Fix default value of primary-reboot-down-after-period in sentinel.conf (valkey-io#1040)

Since in here the monitor value is mymaster, we need to make sure the
primary name is the same, otherwise the default configuration cannot start
sentinel.
```
sentinel monitor mymaster 127.0.0.1 6379 2
```

The following error occurs when the default configuration is started:
```
*** FATAL CONFIG FILE ERROR (Version 255.255.255) ***
Reading the configuration file, at line 358
>>> 'SENTINEL primary-reboot-down-after-period myprimary 0'
No such master with specified name.
```

Introduced in valkey-io#647.

Signed-off-by: Binbin <binloveplay1314@qq.com>

Author: enjoy-binbin

sentinel.conf

@@ -355,4 +355,4 @@ SENTINEL announce-hostnames no
 # accept a -LOADING response after a primary has been rebooted, before failing
 # over.
 
-SENTINEL primary-reboot-down-after-period myprimary 0
+SENTINEL primary-reboot-down-after-period mymaster 0