boards: st: nucleo_h7 boards correct LD2 on PE1 pin

The stm32h7 nucleo 144 boards (ref MB1364)
have the LD2 on PE1 pin (not PB7). This commit change this
in the doc. Board DTS are correct.

Signed-off-by: Francois Ramu <francois.ramu@st.com>