Update deep-rnn.md (d2l-ai#2332)

* Update deep-rnn.md

I will received error `IndexError: list index out of range` while using the previous code.

* Update chapter_recurrent-modern/deep-rnn.md

Co-authored-by: Aston Zhang <22279212+astonzhang@users.noreply.github.com>

chapter_recurrent-modern/deep-rnn.md

@@ -153,7 +153,7 @@ layer by layer.
 @d2l.add_to_class(StackedRNNScratch)
 def forward(self, inputs, Hs=None):
     outputs = inputs
-    if Hs is None: Hs = [None] * len(inputs)
+    if Hs is None: Hs = [None] * self.num_layers
     for i in range(self.num_layers):
         outputs, Hs[i] = self.rnns[i](outputs, Hs[i])
     return outputs, Hs