Fix dart_tools typos (#3042)

diff --git a/lib/src/lock_file.dart b/lib/src/lock_file.dart
index c9377e8..bdfe30f 100644
--- a/lib/src/lock_file.dart
+++ b/lib/src/lock_file.dart
@@ -228,7 +228,7 @@
   }) {
     var header = '''
 This file is deprecated. Tools should instead consume 
-`.dart_tools/package_config.json`.
+`.dart_tool/package_config.json`.
 
 For more info see: https://dart.dev/go/dot-packages-deprecation
 
@@ -249,7 +249,7 @@
     return text.toString();
   }
 
-  /// Returns the contents of the `.dart_tools/package_config` file generated
+  /// Returns the contents of the `.dart_tool/package_config` file generated
   /// from this lockfile.
   ///
   /// This file will replace the `.packages` file.