Address some documentation errors

Author: wu-hui

firebase-firestore/src/main/java/com/google/firebase/firestore/DocumentSnapshot.java

@@ -486,10 +486,10 @@ public DocumentReference getReference() {
 
   /**
    * Returns the value of the field as a {@link VectorValue} or
-   * `null` if the field does not exist in the document.
+   * {@code null} if the field does not exist in the document.
    *
    * @param field The path to the field.
-   * @throws RuntimeException if the value is not a VectorValue.
+   * @throws RuntimeException if the value is not a {@code VectorValue}.
    * @return The value of the field.
    */
   @Nullable