fix: do not delete last element if target index is out of range

SliceTricks.md

@@ -49,9 +49,9 @@ a = a[:len(a)-j+i]
 ```go
 if i < len(a)-1 {
   copy(a[i:], a[i+1:])
-}
   a[len(a)-1] = nil // or the zero value of T
   a = a[:len(a)-1]
+}
 ```
 
 > **Delete without preserving order**