doc: efr32_slwstk6061a: update paragraph 'Flashing'

Zephyr SDK 0.9.4 added OpenOCD support for SiLabs EFR32FG1P SoC. This
commit updates 'Flashing' paragraph of the board documentation to
reflect this change.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>
1 file changed