Fix serde visitor for byts with use_avro_rs_unions option

[mati865]

NOTE: I don't know how to really test if the deserialization is successful but at least it builds now.

Sorry, missed that in #69 and it results in this error when enabling use_avro_rs_unions:

error: expected one of `->`, `<`, `where`, or `{`, found `[`
   --> <generated_file>:100:22
    |
86  |         impl<'de> serde::de::Visitor<'de> for UnionStringBytesVisitor {
    |                                                                       - while parsing this item list starting here
...
100 |             fn visit_[u8]<E>(self, value: &[u8]) -> Result<Self::Value, E>
    |                      ^ expected one of `->`, `<`, `where`, or `{`
...
106 |         }
    |         - the item list ends here

[lerouxrgd]

Thanks, nice catch !