tests: drivers: flash: Renamed missed board rename

Fixes a test which missed the rename of a board

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

tests/drivers/flash/common/testcase.yaml

@@ -45,7 +45,7 @@ tests:
     platform_allow:
       - nucleo_f103rb
       - nucleo_f207zg
-      - stm32f3_disco
+      - stm32f3_disco@B
       - nucleo_f429zi
       - stm32f746g_disco
       - nucleo_f767zi