docs: fix spelling errors in retry (#131)

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Fixes #130 🦕

google/api_core/retry.py

@@ -230,9 +230,9 @@ class Retry(object):
     Args:
         predicate (Callable[Exception]): A callable that should return ``True``
             if the given exception is retryable.
-        initial (float): The minimum a,out of time to delay in seconds. This
+        initial (float): The minimum amount of time to delay in seconds. This
             must be greater than 0.
-        maximum (float): The maximum amout of time to delay in seconds.
+        maximum (float): The maximum amount of time to delay in seconds.
         multiplier (float): The multiplier applied to the delay.
         deadline (float): How long to keep retrying in seconds. The last sleep
             period is shortened as necessary, so that the last retry runs at