docs: fix typo in read_and_write.rst (#3479)

Correct minor typo in docs around using `alter_columns` to type cast.
According to
45c158d
`type` should be `data_type`.

Author: ascillitoe

docs/read_and_write.rst

@@ -300,7 +300,7 @@ at the cost of lower precision:
     })
     dataset = lance.write_dataset(table, "embeddings")
     dataset.alter_columns({"path": "embedding",
-                           "type": pa.list_(pa.float16(), 128)})
+                           "data_type": pa.list_(pa.float16(), 128)})
     dataset.schema()
 
 .. testoutput::