Correct typo

Author: schneiderfelipe

pandas_datareader/av/__init__.py

@@ -28,7 +28,7 @@ def __init__(self, symbols=None, start=None, end=None, retry_count=3,
         if not api_key or not isinstance(api_key, str):
             raise ValueError('The AlphaVantage API key must be provided '
                              'either through the api_key variable or '
-                             'through the environment varaible '
+                             'through the environment variable '
                              'ALPHAVANTAGE_API_KEY')
         self.api_key = api_key