Add missing backtick in keyboard_layout.rs doc comment #978

Job	Run time
pre-commit (check-toml)	51s
pre-commit (check-yaml)	57s
pre-commit (check-merge-conflict)	55s
pre-commit (check-case-conflict)	52s
pre-commit (detect-private-key)	50s
pre-commit (mixed-line-ending)	49s
pre-commit (trailing-whitespace)	51s
pre-commit (cspell)	1m 6s
pre-commit (typos)	53s
pre-commit (cargo-fmt)	49s
pre-commit (cargo-clippy)	2m 14s
pre-commit (cargo-test)	2m 39s
pre-commit (verify_icon_files)	46s
	14m 32s

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