StringBuilder is actually referenced in Part ten section 3.

BenAttenborough · BenAttenborough · commit 9515f439e521 · 2020-07-26T21:05:53.000+01:00
diff --git a/data/part-12/4-multidimensional-data.md b/data/part-12/4-multidimensional-data.md
@@ -112,7 +112,7 @@ Create in the exercise base a method called `public static String arrayAsString(
 
 <!-- Kertaa yhdeksännestä osasta StringBuilderin käyttö ennen tehtävän tekoa. Alla muutamia esimerkkejä metodin odotetusta toiminnasta. -->
 
-Brush up on using StringBuilder in part nine before taking on this exercise. Below there are a few examples of how the method is expected to  work.
+Brush up on using StringBuilder in part 10.3 before taking on this exercise. Below there are a few examples of how the method is expected to  work.
 
 <!-- ```java
 int rows = 2;
