Fix minor typo (#5072)

I believe the sentence `Retrieve AN file..." should read "Retrieve A file...". This is similar to the deletion sentence a few lines later which uses the form "Delete A file..." instead of "Delete AN file".

Author: ericmutta

xml/System.IO.Compression/ZipFile.xml

@@ -48,7 +48,7 @@
 |Create a zip archive from a directory|<xref:System.IO.Compression.ZipFile.CreateFromDirectory%2A?displayProperty=nameWithType>|  
 |Extract the contents of a zip archive to a directory|<xref:System.IO.Compression.ZipFile.ExtractToDirectory%2A?displayProperty=nameWithType>|  
 |Add new files to an existing zip archive|<xref:System.IO.Compression.ZipArchive.CreateEntry%2A?displayProperty=nameWithType>|  
-|Retrieve an file in a zip archive|<xref:System.IO.Compression.ZipArchive.GetEntry%2A?displayProperty=nameWithType>|  
+|Retrieve a file in a zip archive|<xref:System.IO.Compression.ZipArchive.GetEntry%2A?displayProperty=nameWithType>|  
 |Retrieve all of the files in a zip archive|<xref:System.IO.Compression.ZipArchive.Entries%2A?displayProperty=nameWithType>|  
 |To open a stream to an individual file contained in a zip archive|<xref:System.IO.Compression.ZipArchiveEntry.Open%2A?displayProperty=nameWithType>|  
 |Delete a file from a zip archive|<xref:System.IO.Compression.ZipArchiveEntry.Delete%2A?displayProperty=nameWithType>|