Add AlternateRepresentation to Metadata article (#1128)

The metadata article curates a list of its child directives, with relevant topic titles.

This adds `AlternateRepresentation` to that list, in the same section as `SupportedLanguage` as they both deal with language representations.

Author: anferbui

Sources/SwiftDocC/Semantics/Metadata/AlternateRepresentation.swift

@@ -12,7 +12,7 @@ import Foundation
 import Markdown
 
 
-/// A directive that configures an alternate language representations of a symbol.
+/// A directive that configures an alternate language representation of a symbol.
 ///
 /// An API that can be called from more than one source language has more than one language representation.
 ///

Sources/docc/DocCDocumentation.docc/DocC.symbols.json

@@ -75,7 +75,7 @@
       "docComment" : {
         "lines" : [
           {
-            "text" : "A directive that configures an alternate language representations of a symbol."
+            "text" : "A directive that configures an alternate language representation of a symbol."
           },
           {
             "text" : ""

Sources/docc/DocCDocumentation.docc/Reference Syntax/API Reference Syntax/Metadata.md

@@ -89,6 +89,7 @@ comment.
 ### Customizing the Languages of an Article
 
 - ``SupportedLanguage``
+- ``AlternateRepresentation``
 
 ### Customizing the Availability Information of a Page