[tool] Fix BuildInfo.packagesPath doc comment (#103785)

Until recently, BuildInfo.packagesPath defaulted to `.packages` however,
the .packages file has been deprecated [1] and is being removed. In
https://github.com/flutter/flutter/pull/99677 the default was changed to
`.dart_tool/package_config.json` but the doc comment was accidentally
updated to `.dart_tool/packages`. This corrects it to the true default
value.

1: https://github.com/dart-lang/sdk/issues/48272

Spotted in the midst of some related cleanup relating to
https://github.com/flutter/flutter/issues/103775

packages/flutter_tools/lib/src/build_info.dart

@@ -70,7 +70,7 @@ class BuildInfo {
   /// The path to the package configuration file to use for compilation.
   ///
   /// This is used by package:package_config to locate the actual package_config.json
-  /// file. If not provided, defaults to `.dart_tool/packages`.
+  /// file. If not provided, defaults to `.dart_tool/package_config.json`.
   final String packagesPath;
 
   final List<String> fileSystemRoots;