DOC: typos spotted by Chris M

Thanks for your eagle eye.

Author: matthew-brett

nibabel/dataobj_images.py

@@ -138,7 +138,7 @@ def get_data(self, caching='fill'):
         (no reference to the array).  If the cache is full, "unchanged" leaves
         the cache full and returns the cached array reference.
 
-        The cache can effect the behavior of the image, because if the cache is
+        The cache can affect the behavior of the image, because if the cache is
         full, or you have an array image, then modifying the returned array
         will modify the result of future calls to ``get_data()``.  For example
         you might do this:

nibabel/tests/test_image_api.py

@@ -208,7 +208,7 @@ def validate_data_interface(self, imaker, params):
                 assert_true(is_proxy(img.dataobj))
                 # Confirm it is not a numpy array
                 assert_false(isinstance(img.dataobj, np.ndarray))
-                # Confirm it can be converted to a numpy array with is_array
+                # Confirm it can be converted to a numpy array with asarray
                 proxy_data = np.asarray(img.dataobj)
                 proxy_copy = proxy_data.copy()
                 # Not yet cached, proxy image: in_memory is False