fix(kotlin): use string format specifier for record line

The appropriate specifier for unsigned integers is `d` but
this triggers an `IllegalFormatConversionException` panic.
With `s`, we can successfully build the nodes with the custom
log writer

Author: enigbe

bindings/kotlin/ldk-node-jvm/lib/src/test/kotlin/org/lightningdevkit/ldknode/LibraryTest.kt

@@ -132,7 +132,7 @@ class CustomLogWriter(private var currentLogLevel: LogLevel = LogLevel.INFO) :
             java.time.LocalDateTime.now()
                 .format(java.time.format.DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss"))
         return String.format(
-            "%s %-5s [%s:%d] %s\n",
+            "%s %-6s [%s:%s] %s\n",
             timestamp,
             record.level,
             record.modulePath,