Use "call" instead of "invoke" (#4177)

For consistency.

Fixes #4142

packages/flutter_tools/lib/src/device.dart

diff --git a/packages/flutter_tools/lib/src/device.dart b/packages/flutter_tools/lib/src/device.dart
index fb8a663..3528682 100644
--- a/packages/flutter_tools/lib/src/device.dart
+++ b/packages/flutter_tools/lib/src/device.dart
@@ -19,7 +19,7 @@
 /// A class to get all available devices.
 class DeviceManager {
   /// Constructing DeviceManagers is cheap; they only do expensive work if some
-  /// of their methods are invoked.
+  /// of their methods are called.
   DeviceManager() {
     // Register the known discoverers.
     _deviceDiscoverers.add(new AndroidDevices());