Improve concurrency example of transient invalid states (#344)

The old example actually makes an atomic update to the array of
temperatures because it uses `map`.  Updating them in a `for` loop
makes it more obvious that some temperatures will be Fahrenheit and
some Celsius when the program's in the middle of the loop.

Fixes: rdar://139633262

Author: amartini51

TSPL.docc/LanguageGuide/Concurrency.md

@@ -1107,8 +1107,8 @@ the code below converts measured temperatures from Fahrenheit to Celsius:
 ```swift
 extension TemperatureLogger {
     func convertFahrenheitToCelsius() {
-        measurements = measurements.map { measurement in
-            (measurement - 32) * 5 / 9
+        for i in measurements.indices {
+            measurements[i] = (measurements[i] - 32) * 5 / 9
         }
     }
 }