doc: log: correct option name CONFIG_LOG_MAX_LEVEL

Replace CONFIG_(SPL_)MAX_LOG_LEVEL by the correct name as defined in
common/Kconfig:
line 668:config LOG_MAX_LEVEL
line 688:config SPL_LOG_MAX_LEVEL
line 708:config TPL_LOG_MAX_LEVEL

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>