SliceTricks: use builtin clear

After filtering without allocating, it is recommended to loop over the
underlying slice a and setting all unneeded values to nil manually if they
are to be garbage collected.

Instead, one might simply use the builtin 'clear' on the sub-slice starting
at the end of b.

Author: TyPh00nCdr

SliceTricks.md

@@ -187,9 +187,7 @@ for _, x := range a {
 For elements which must be garbage collected, the following code can be included afterwards:
 
 ```go
-for i := len(b); i < len(a); i++ {
-	a[i] = nil // or the zero value of T
-}
+clear(a[len(b):])
 ```
 
 ### Reversing