Add a missing quote in DebugLogbackConfigurator

See gh-17357

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/logging/logback/DebugLogbackConfigurator.java

@@ -40,7 +40,7 @@ class DebugLogbackConfigurator extends LogbackConfigurator {
 	@Override
 	@SuppressWarnings("rawtypes")
 	public void conversionRule(String conversionWord, Class<? extends Converter> converterClass) {
-		info("Adding conversion rule of type '" + converterClass.getName() + "' for word '" + conversionWord);
+		info("Adding conversion rule of type '" + converterClass.getName() + "' for word '" + conversionWord + "'");
 		super.conversionRule(conversionWord, converterClass);
 	}