fixing doc typos

I started fixing typos when I noticed this mistake in the current version of the docs:

```
If the conditions are met, the master just sends the part of the replication stream the master missed, and the replication continues. Otherwise a full resynchronization is performed as in the past versions of Redis.
```

That should have read "the part of the replication stream the slave missed". I fixed that mistake, and cleaned up some wording and typos on the rest of the page.