Fixed compilation error/typo (#20933)

* Fixed compilation error

Variable not named. Corrected it to match.

* Fixed formatting

guide/english/csharp/return/index.md

@@ -21,7 +21,7 @@ class Calc
   {
     int a = 4;
     int b = 3;
-    int sum = Sum(a, b);
+    int result = Sum(a, b);
     Console.WriteLine($"The sum of {a} and {b} is {result}");
     
     // To keep the console from closing
@@ -35,4 +35,4 @@ class Calc
 ## Output:
 ```
 > The sum of 4 and 3 is 7
-``
+```