back.rtlil: espace { and } in format string #3570

Job	Run time
check-links	43s
document	43s
smoke (amaranth-lang/amaranth-boards)	33s
smoke (amaranth-lang/amaranth-stdio)	39s
test (3.8)	2m 15s
smoke (amaranth-lang/amaranth-soc)	40s
test (3.9)	2m 16s
test (3.10)	2m 13s
test (3.11)	2m 39s
test (3.12)	3m 26s
test (pypy-3.8)	5m 3s
test (pypy-3.9)	4m 57s
test (pypy-3.10)	4m 50s
test (3.13-dev)	3m 4s
publish-docs	6s
publish-docs-dev	0s
required	1s
publish-package	40s
publish-schemas	5s
publish-release	0s
	34m 53s

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