Add missing "a" to a qldoc comment

Co-authored-by: intrigus-lgtm <60750685+intrigus-lgtm@users.noreply.github.com>

java/ql/lib/semmle/code/java/Type.qll

@@ -710,7 +710,7 @@ class Class extends ClassOrInterface {
   }
 
   /**
-   * Holds if this class is Kotlin "file class", e.g. the class FooKt
+   * Holds if this class is a Kotlin "file class", e.g. the class FooKt
    * for top-level entities in Foo.kt.
    */
   predicate isFileClass() { file_class(this) }