Replaces the converter argument value SINGLE_VALUE with string as it makes more sense

src/main/java/net/knarcraft/ffmpegconverter/parser/ConverterArgumentValue.java

@@ -3,6 +3,6 @@ package net.knarcraft.ffmpegconverter.parser;
 public enum ConverterArgumentValue {
     BOOLEAN,
     COMMA_SEPARATED_LIST,
-    SINGLE_VALUE,
+    STRING,
     INT
 }