Fix typo (#4579)

Author: mairaw

xml/System.Text/Encoding.xml

@@ -1123,7 +1123,7 @@ In the .NET Framework on the Windows desktop, the <xref:System.Text.Encoding.Def
 
 ## The Default property on .NET Core
 
-On .NET Core, the <xref:System.Text.Encoding.Default%2A> property always returns the <xref:System.Text.UTF8Encoding>. UTF-8 is supported on all the operating systems (Windows, Linux, and Max OS X) on which .NET Core applications run.
+On .NET Core, the <xref:System.Text.Encoding.Default%2A> property always returns the <xref:System.Text.UTF8Encoding>. UTF-8 is supported on all the operating systems (Windows, Linux, and macOS) on which .NET Core applications run.
 
  ]]></format>
         </remarks>