Fix grammar in FlBinaryCodec/FlStringCodec descriptions (#18268)

diff --git a/shell/platform/linux/public/flutter_linux/fl_binary_codec.h b/shell/platform/linux/public/flutter_linux/fl_binary_codec.h
index 9b444fc..8e5e0c1 100644
--- a/shell/platform/linux/public/flutter_linux/fl_binary_codec.h
+++ b/shell/platform/linux/public/flutter_linux/fl_binary_codec.h
@@ -22,7 +22,7 @@
 /**
  * FlBinaryCodec:
  *
- * #FlBinaryCodec is a #FlMessageCodec that implements the Flutter binary
+ * #FlBinaryCodec is an #FlMessageCodec that implements the Flutter binary
  * message encoding. This encodes and decodes #FlValue of type
  * #FL_VALUE_TYPE_UINT8_LIST.
  *
diff --git a/shell/platform/linux/public/flutter_linux/fl_string_codec.h b/shell/platform/linux/public/flutter_linux/fl_string_codec.h
index 948760d..8778046 100644
--- a/shell/platform/linux/public/flutter_linux/fl_string_codec.h
+++ b/shell/platform/linux/public/flutter_linux/fl_string_codec.h
@@ -22,7 +22,7 @@
 /**
  * FlStringCodec:
  *
- * #FlStringCodec is a #FlMessageCodec that implements the Flutter string
+ * #FlStringCodec is an #FlMessageCodec that implements the Flutter string
  * message encoding. This encodes and decodes #FlValue of type
  * #FL_VALUE_TYPE_STRING.
  *