bank-account: fix

Author: ErikSchierboom

exercises/practice/bank-account/.meta/Generator.tpl

@@ -51,7 +51,7 @@ public class {{ testClass }}
                         {{- end -}}
                     }
                 }));
-                await Task.WaitAll(tasks.ToArray());
+                await Task.WhenAll(tasks.ToArray());
             }
             {{- else }}
             account.{{ op | to_call }};

exercises/practice/bank-account/BankAccountTests.cs

@@ -168,7 +168,7 @@ public async Task Can_handle_concurrent_transactions()
                     account.Withdraw(1m);
                 }
             }));
-            await Task.WaitAll(tasks.ToArray());
+            await Task.WhenAll(tasks.ToArray());
         }
         Assert.Equal(0m, account.Balance);
     }