[bluetooth] fix incorrect BlueZ version in comment

The "type" option of the GATT Characteristic.WriteValue method was
actually introduced in BlueZ v5.51 (as can be seen in the link below).

https://github.com/bluez/bluez/commit/aa9f9b193c51d371b5d2a863f35a034ee0377b22

Change-Id: I0f7513582d91c98446d06148d1edbe9e2502ee44
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2214411Reviewed-by: Sonny Sasaka <sonnysasaka@chromium.org>
Commit-Queue: Sonny Sasaka <sonnysasaka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771857}

device/bluetooth/dbus/bluetooth_gatt_characteristic_client.cc

@@ -133,7 +133,7 @@ class BluetoothGattCharacteristicClientImpl
     // Append option dict
     base::DictionaryValue dict;
     if (!type_option.empty()) {
-      // NB: the "type" option was added in BlueZ 5.50. Older versions of BlueZ
+      // NB: the "type" option was added in BlueZ 5.51. Older versions of BlueZ
       // will ignore this option.
       dict.SetStringKey(bluetooth_gatt_characteristic::kOptionType,
                         type_option);