Update exercises/105_threading2.zig

The last word, '"diggits" was misspelled.

exercises/105_threading2.zig

@@ -104,4 +104,4 @@ fn thread_pi(pi: *f64, begin: u64, end: u64) !void {
 // to such an extent that seconds become minutes during execution.
 //
 // And you should remove the formatting restriction in "print",
-// otherwise you will not be able to see the additional diggits.
+// otherwise you will not be able to see the additional digits.