Documentation:Fix Seeed Studio XIAO nRF52840

Change XIAO nRF52840 name at Documentation to
match product name and other Seeed Studio boards.

Signed-off-by: Rodrigo Sim <rcsim10@gmail.com>

Author: rcsim

Documentation/platforms/arm/nrf52/boards/xiao-nrf52840/index.rst

@@ -1,6 +1,6 @@
-=============
-XIAO nRF52840
-=============
+==========================
+Seeed Studio XIAO nRF52840
+==========================
 
 The `Seeed Studio XIAO nRF52840 <https://wiki.seeedstudio.com/XIAO_BLE/>`_ is a general purpose board supplied by
 Seeed Studio and it is compatible with the Nordic nRF52840 ecosystem as they share the same MCU.