Fixed link in ledring with cf21-bl doc #1958

Job	Run time
read_targets_from_file / read_targets_from_file	5s
basic_build (cf2)	3m 30s
basic_build (bolt)	3m 53s
basic_build (tag)	3m 29s
basic_build (flapper)	3m 31s
basic_build (cf21bl)	3m 44s
kbuild-targets (allyesconfig)	3m 29s
features (bosch.conf)	3m 46s
kbuild-targets (allnoconfig)	3m 22s
features (loco_tdma.conf)	3m 36s
kbuild-targets (randconfig)	3m 29s
apps (examples/app_hello_world)	3m 0s
features (loco_tdoa2.conf)	3m 29s
apps (examples/app_hello_world-cpp)	2m 51s
features (loco_tdoa3.conf)	3m 31s
apps (examples/app_hello_file_tree)	3m 1s
features (loco_tdoa3_all.conf)	3m 27s
apps (examples/app_peer_to_peer)	3m 2s
features (bigquad.conf)	3m 28s
apps (examples/app_p2p_DTR)	3m 1s
features (app_api.conf)	3m 31s
apps (examples/app_stm_gap8_cpx)	3m 0s
apps (examples/demos/app_push_demo)	2m 52s
apps (examples/demos/swarm_demo)	2m 57s
apps (examples/demos/app_wall_following_demo)	2m 53s
	1h 19m 57s

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