back.rtlil: espace { and } in format string #3571

Job	Run time
check-links	45s
smoke (amaranth-lang/amaranth-boards)	36s
document	44s
test (3.8)	2m 24s
smoke (amaranth-lang/amaranth-stdio)	37s
smoke (amaranth-lang/amaranth-soc)	40s
test (3.9)	2m 26s
test (3.10)	2m 20s
test (3.11)	2m 52s
test (3.12)	3m 36s
test (pypy-3.8)	5m 1s
test (pypy-3.9)	5m 2s
test (pypy-3.10)	5m 54s
test (3.13-dev)	3m 10s
publish-docs	14s
publish-docs-dev	0s
required	1s
publish-package	51s
publish-schemas	13s
publish-release	0s
	37m 26s

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