lr_finder: Fix typo in docstring (Lightning-AI#1746)

Author: xerus

pytorch_lightning/trainer/lr_finder.py

@@ -94,7 +94,7 @@ def lr_find(self,
 
             # Inspect results
             fig = lr_finder.plot(); fig.show()
-            suggested_lr = lr_finder.suggest()
+            suggested_lr = lr_finder.suggestion()
 
             # Overwrite lr and create new model
             hparams.lr = suggested_lr