fix: test case for interest_is_interesting_test.cpp

The test case titled "Years before large difference between start and target balance" contains an incorrect `want`.

Currently, `want == 85`, but if we substitute this into the expressions `balance * (1 + (interest_rate / 100))^want` and `balance * (1 + (interest_rate / 100))^(want + 1)`, we see that both expressions have a value that is much lower than `target_balance`.

This commit fixes this by setting `want` to the correct value.

Author: mhdadk

exercises/concept/interest-is-interesting/interest_is_interesting_test.cpp

@@ -169,7 +169,7 @@ TEST_CASE("Years before desired balance for large start balance", "[task_4]") {
 TEST_CASE("Years before large difference between start and target balance", "[task_4]") {
     double balance{2345.67};
     double target_balance{12345.6789};
-    int want{85};
+    int want{104};
     REQUIRE(years_until_desired_balance(balance, target_balance) == want);
 }
 TEST_CASE("Balance is already above target", "[task_4]") {