Updated metrics/classification/precision_recall.py (#5348)

There was a typo in Documentation of Code of the ```compute()``` function of ```Recall``` metric at line 210. It said "Computes accuracy over state." which should have been "Computes recall over state."

(cherry picked from commit d568533)

Author: abhik-99

pytorch_lightning/metrics/classification/precision_recall.py

@@ -207,7 +207,7 @@ def update(self, preds: torch.Tensor, target: torch.Tensor):
 
     def compute(self):
         """
-        Computes accuracy over state.
+        Computes recall over state.
         """
         if self.average == 'micro':
             return self.true_positives.sum().float() / (self.actual_positives.sum() + METRIC_EPS)