Documentation for write_i8 in async_write_ext.rs should use signed integers

[grydholt]

Version
tokio v1.21.1

The documentation for write_i8 in io/util/async_write_ext.rs states

           /// Writes an unsigned 8-bit integer to the underlying writer.

This seems to be a copy-paste error from write_u8. The code example also uses write_u8 instead of write_i8.