fix typo

repo2docker/buildpacks/julia.py

@@ -16,7 +16,7 @@ class JuliaBuildPack(CondaBuildPack):
         """Get additional environment settings for Julia and Jupyter
 
         Returns:
-            an ordered list of environment setting tuples.
+            An ordered list of environment setting tuples.
             The tuple contains a string of the environment variable name and
             a string of the environment setting. For example,
             `('JULIA_VERSION', '0.6.0')`