[stm32] fix dox for rcc_periph_x

Seems the dox for rcc_periph_reset_pulse and some others were wrong.
You should use it as ```rcc_periph_reset_pulse(RST_I2C1)``` instead of ```rcc_periph_reset_pulse(RCC_I2C1)``` as the dox previously suggested...

Would be nice if someone can verify this.