Fix an incomplete comment.

R=jmesserly@google.com

Review URL: https://codereview.chromium.org//1993023002 .

lib/src/runner/configuration.dart

@@ -311,7 +311,8 @@ class Configuration {
         addTags: addTags,
 
         // Make sure we pass [chosenPresets] to the child configurations as
-        // well. This ensures that 
+        // well. This ensures that tags and platforms can have preset-specific
+        // behavior.
         tags: _withChosenPresets(tags, chosenPresets),
         onPlatform: _withChosenPresets(onPlatform, chosenPresets),
         presets: _withChosenPresets(presets, chosenPresets));