psbt2: Fix field number counting

PSBT_OUT_SCRIPT should be 0x04, not 0x03.

Author: achow101

bip-0174.mediawiki

@@ -501,7 +501,7 @@ determine which outputs are change outputs and verify that the change is returni
 | [[bip-psb2.mediawiki|psbt2]]
 |-
 | Output Script
-| <tt>PSBT_OUT_SCRIPT = 0x03</tt>
+| <tt>PSBT_OUT_SCRIPT = 0x04</tt>
 | None
 | No key data
 | <tt><script></tt>

bip-0370.mediawiki

@@ -197,7 +197,7 @@ The new per-output types for PSBT Version 2 are defined as follows:
 | 2
 |-
 | Output Script
-| <tt>PSBT_OUT_SCRIPT = 0x03</tt>
+| <tt>PSBT_OUT_SCRIPT = 0x04</tt>
 | None
 | No key data
 | <tt><script></tt>