fix: correct typo in Serializer doc comment

[maxtaran2010]

The doc comment on the Serializer struct incorrectly says "deserializing" when it should say "serializing". The to_string function is a serialization function, not a deserialization function.

Before:
```rust
/// You can just use [to_string] for deserializing a value.
```

After:
```rust
/// You can just use [to_string] for serializing a value.
```

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[juntyr]

Thanks for catching the error and providing the fix @maxtaran2010!