specify PSBT_IN_PREVIOUS_TXID serialization order

Author: Shadouts

bip-0174.mediawiki

@@ -389,7 +389,7 @@ The currently defined per-input types are defined as follows:
 | None
 | No key data
 | <tt><32 byte txid></tt>
-| 32 byte txid of the previous transaction whose output at PSBT_IN_OUTPUT_INDEX is being spent.
+| 32 byte little endian txid of the previous transaction whose output at PSBT_IN_OUTPUT_INDEX is being spent.
 | 2
 | 0
 | 2

bip-0370.mediawiki

@@ -127,7 +127,7 @@ The new per-input types for PSBT Version 2 are defined as follows:
 | None
 | No key data
 | <tt><32 byte txid></tt>
-| 32 byte txid of the previous transaction whose output at PSBT_IN_OUTPUT_INDEX is being spent.
+| 32 byte little endian txid of the previous transaction whose output at PSBT_IN_OUTPUT_INDEX is being spent.
 | 2
 | 0
 | 2