PSBT: fix phpdoc for function returning unknown key/values

Author: Thomas Kerin

src/Transaction/PSBT/PSBT.php

@@ -167,8 +167,9 @@ public function getTransaction(): TransactionInterface
     {
         return $this->tx;
     }
+
     /**
-     * @return string[]
+     * @return BufferInterface[]
      */
     public function getUnknowns(): array
     {

src/Transaction/PSBT/PSBTInput.php

@@ -373,6 +373,9 @@ public function getFinalizedScriptWitness(): ScriptWitnessInterface
         return $this->finalScriptWitness;
     }
 
+    /**
+     * @return BufferInterface[]
+     */
     public function getUnknownFields(): array
     {
         return $this->unknown;

src/Transaction/PSBT/PSBTOutput.php

@@ -161,7 +161,7 @@ public function getBip32Derivations(): array
     }
 
     /**
-     * @return string[]
+     * @return BufferInterface[]
      */
     public function getUnknownFields(): array
     {