back.rtlil: espace { and } in format string #3573

Job	Run time
smoke (amaranth-lang/amaranth-boards)	38s
test (3.8)	2m 34s
smoke (amaranth-lang/amaranth-stdio)	43s
document	50s
check-links	43s
test (3.9)	2m 26s
smoke (amaranth-lang/amaranth-soc)	41s
test (3.10)	2m 18s
test (3.11)	2m 55s
test (3.12)	3m 29s
test (pypy-3.8)	5m 26s
test (pypy-3.9)	5m 5s
test (pypy-3.10)	4m 59s
test (3.13-dev)	3m 5s
publish-docs	5s
publish-docs-dev	0s
required	1s
publish-schemas	9s
publish-package	38s
publish-release	0s
	36m 45s

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