Update schema.md

Change datum "first used" to "first observed"

Author: WubbleWobble

doc/schema.md

@@ -564,7 +564,7 @@ A table containing Plutus Datum, found in witnesses or inlined in outputs
 |-|-|-|
 | `id` | integer (64) |  |
 | `hash` | hash32type | The Hash of the Datum |
-| `tx_id` | integer (64) | The Tx table index for the transaction where this datum was first used. |
+| `tx_id` | integer (64) | The Tx table index for the transaction where this datum was first observed. |
 | `value` | jsonb | The actual data in JSON format (detailed schema) |
 | `bytes` | bytea | The actual data in CBOR format |