Fix deprecated use of ${var} in Encoding.php

Since PHP 8.2 the syntax "${var}" is deprecated.
"{$var}" should be used instead.

src/AWS/CRT/Internal/Encoding.php

@@ -24,7 +24,7 @@ public static function readStrings($buffer) {
     public static function decodeString($buffer) {
         $len = unpack("N", $buffer)[1];
         $buffer = substr($buffer, 4);
-        $str = unpack("a${len}", $buffer)[1];
+        $str = unpack("a{$len}", $buffer)[1];
         return [$len, $str];
     }