docs: Include INFLUXDB_IOX_OBJECT_STORE in env.example
# Must be a number that can be represented by a 32-bit unsigned integer.
# INFLUXDB_IOX_ID=1
#
+# Which object store implementation to use (defaults to Memory if unset)
+# INFLUXDB_IOX_OBJECT_STORE=File
+#
# Where to store files on disk:
# INFLUXDB_IOX_DB_DIR=$HOME/.influxdb_iox
# TEST_INFLUXDB_IOX_DB_DIR=$HOME/.influxdb_iox
