Add heading attribute to glossary.md #4928

Job	Run time
format	33s
typos	4s
cargo (ubuntu-latest)	48s
cargo (macos-latest)	58s
bare-metal (src/bare-metal/alloc-example, aarch64-unknown-none)	36s
find-languages	3s
bare-metal (src/bare-metal/aps/examples, aarch64-unknown-none)	29s
bare-metal (src/bare-metal/microcontrollers/examples, thumbv7em-none-eabihf)	45s
bare-metal (src/exercises/bare-metal/compass, thumbv7em-none-eabihf)	47s
bare-metal (src/exercises/bare-metal/rtc, aarch64-unknown-none)	33s
build (bn)	2m 42s
build (da)	2m 44s
build (de)	2m 38s
build (el)	2m 46s
build (en)	2m 29s
build (es)	2m 37s
build (fa)	2m 40s
build (fr)	2m 41s
build (id)	2m 38s
build (it)	2m 37s
build (ja)	2m 34s
build (ko)	2m 33s
build (pl)	2m 36s
build (pt-BR)	2m 32s
build (ro)	2m 37s
build (ru)	2m 37s
build (tr)	2m 40s
build (uk)	2m 52s
build (zh-CN)	2m 42s
build (zh-TW)	2m 24s
	58m 15s

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