Improve Zip archives documentation grammar

As the format is `Zip`, it's wrong to precede it with an article `a` which would be more appropriate for an instance of the format.

Author: jsoref

README.md

@@ -451,7 +451,7 @@ You may also be limited by Artifacts if you have exceeded your shared storage qu
 
 ### Zip archives
 
-When an Artifact is uploaded, all the files are assembled into an immutable Zip archive. There is currently no way to download artifacts in a format other than a Zip or to download individual artifact contents.
+When an Artifact is uploaded, all the files are assembled into an immutable Zip archive. There is currently no way to download artifacts in a format other than Zip or to download individual artifact contents.
 
 ### Permission Loss