[1.5.x] Fixed spelling mistake in file docs.

Backport of a3e88e64a4 from master

docs/howto/custom-file-storage.txt

@@ -90,7 +90,7 @@ to the ``get_valid_name()`` method described above.
     extension.
 
     Previously, an underscore followed by a number (e.g. ``"_1"``, ``"_2"``,
-    etc.) was appended to the filename until an avaible name in the destination
+    etc.) was appended to the filename until an available name in the destination
     directory was found. A malicious user could exploit this deterministic
     algorithm to create a denial-of-service attack. This change was also made
     in Django 1.4.14.

docs/ref/files/storage.txt

@@ -88,7 +88,7 @@ The Storage Class
         extension.
 
         Previously, an underscore followed by a number (e.g. ``"_1"``, ``"_2"``,
-        etc.) was appended to the filename until an avaible name in the
+        etc.) was appended to the filename until an available name in the
         destination directory was found. A malicious user could exploit this
         deterministic algorithm to create a denial-of-service attack. This
         change was also made in Django 1.4.14.