docs: fix fomatting of siggen man page #11005

Job	Run time
rip-and-test	59m 1s
package-arch (debian:bullseye)	28m 45s
package-arch (debian:bookworm)	29m 38s
package-indep (debian:bullseye)	42m 21s
htmldocs	31m 11s
package-arch (debian:sid)	30m 6s
package-indep (debian:bookworm)	32m 57s
rip-and-test-clang	51m 49s
package-indep (debian:sid)	37m 34s
	5h 43m 22s

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