Update rnn.md

Author: sandra-haerin-ha

rnn.md

@@ -196,3 +196,7 @@ are trained jointly, and the diagram in Figure 4 represents one computational gr
 So far we have seen only a simple recurrence formula for the Vanilla RNN. In practice, we actually will
 rarely ever use Vanilla RNN formula. Instead, we will use what we call a Long-Short Term Memory (LSTM)
 RNN.
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+### Gradient Flow 
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+### Vanishing gradient problem