[guide] improve ++/-- example.

Fixes #1130.

README.md

@@ -1521,24 +1521,28 @@ Other Style Guides
 
       let array = [1, 2, 3];
       let num = 1;
-      let increment = num ++;
-      let decrement = -- num;
+      num ++;
+      -- num;
 
+      let sum = 0;
+      let truthyCount = 0;
       for(let i = 0; i < array.length; i++){
         let value = array[i];
-        ++value;
+        sum += value;
+        if (value) {
+          truthyCount++;
+        }
       }
 
       // good
 
       let array = [1, 2, 3];
       let num = 1;
-      let increment = num += 1;
-      let decrement = num -= 1;
+      num += 1;
+      num -= 1;
 
-      array.forEach((value) => {
-        value += 1;
-      });
+      const sum = array.reduce((a, b) => a + b, 0);
+      const truthyCount = array.filter(Boolean).length;
     ```
 
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