env: Kconfig: cosmetics: update comment for SYS_RELOC_GD_ENV_ADDR

Update the comment for SYS_RELOC_GD_ENV_ADDR as gd->env_addr is updated
in board_r.c::initr_reloc_global_data() under the compilation flags
CONFIG_SYS_RELOC_GD_ENV_ADDR

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Tom Rini <trini@konsulko.com>