[Doc] Fix unescaped reference in 'Substring.base' summary.

There is an unescaped reference to type `Substring`.  I think a plain lowercased reference would read better.  Alternatively we could escape it with backquotes.

Author: rxwei

stdlib/public/core/Substring.swift

@@ -122,7 +122,7 @@ public struct Substring: Sendable {
 }
 
 extension Substring {
-  /// Returns the underlying string from which this Substring was derived.
+  /// Returns the underlying string from which this substring was derived.
   @_alwaysEmitIntoClient
   public var base: String { return _slice.base }