Make .enumerate() example self-explanatory

src/doc/book/loops.md

@@ -105,19 +105,19 @@ When you need to keep track of how many times you already looped, you can use th
 #### On ranges:
 
 ```rust
-for (i, j) in (5..10).enumerate() {
-    println!("i = {} and j = {}", i, j);
+for (index, value) in (5..10).enumerate() {
+    println!("index = {} and value = {}", index, value);
 }
 ```
 
 Outputs:
 
 ```text
-i = 0 and j = 5
-i = 1 and j = 6
-i = 2 and j = 7
-i = 3 and j = 8
-i = 4 and j = 9
+index = 0 and value = 5
+index = 1 and value = 6
+index = 2 and value = 7
+index = 3 and value = 8
+index = 4 and value = 9
 ```
 
 Don't forget to add the parentheses around the range.