Fix OS Tuning section code blocks
Fix OS Tuning section code blocks

Running `sysctl -w vm.dirty_ratio = 80` giving following error

```
sysctl: "vm.dirty_ratio" must be of the form name=value
sysctl: malformed setting "="
sysctl: "80" must be of the form name=value
```

Changed command with correct structure

Syntax reference: http://man7.org/linux/man-pages/man8/sysctl.8.html
deniszh/go-carbon,deniszh/go-carbon,lomik/go-carbon,deniszh/go-carbon,lomik/go-carbon,lomik/go-carbon