Fix exaö.e WeakRef example by removing ref.value call. (#10846)

The `ref.value` call cause the GC to not collect `ref` until end of scope.

Co-authored-by: Beta Ziliani <[email protected]>

src/weak_ref.cr

@@ -7,10 +7,14 @@
 # require "weak_ref"
 #
 # ref = WeakRef.new("oof".reverse)
-# p ref.value # => "foo"
+# p ref # => #<WeakRef(String):0x7f83406eafa0 @target=Pointer(Void)@0x7f83406eafc0>
 # GC.collect
+# p ref # => #<WeakRef(String):0x7f83406eafa0 @target=Pointer(Void).null>
 # p ref.value # => nil
 # ```
+#
+# Note that the collection of objects is not deterministic, and depends on many subtle aspects. For instance,
+# if the example above is modified to print `ref.value` in the first print, then the collector will not collect it.
 class WeakRef(T)
  @target : Void*