Fix formatting of warning block in `README.md` #456

Job	Run time
Alpine Linux	2m 27s
FreeBSD-x86_64 with rust stable	39s
Linux-x86_64 with rust stable	1m 25s
Linux-x86_64 (rustls-native-roots) with rust stable	1m 7s
Linux-x86_64 (native-tls) with rust stable	1m 29s
Linux-x86_64 (native-tls-vendored) with rust stable	2m 38s
Linux-aarch64 with rust stable	1m 24s
Linux-arm with rust stable	1m 15s
Linux-i686 with rust stable	41s
Linux-powerpc with rust stable	46s
Linux-powerpc64 with rust stable	47s
Linux-powerpc64le with rust stable	49s
Linux-riscv64 with rust stable	1m 14s
Linux-s390x with rust stable	54s
NetBSD-x86_64 with rust stable	2m 7s
Windows-aarch64 with rust stable	2m 43s
Windows-i686 with rust stable	3m 33s
Windows-x86_64 with rust stable	5m 5s
macOS-x86_64 with rust stable	1m 53s
macOS-aarch64 with rust stable	1m 16s
Linux-x86_64 with rust beta	3m 34s
Linux-x86_64 with rust nightly	3m 21s
	41m 7s

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