Fix formatting of warning block in `README.md` #457

Job	Run time
Alpine Linux	2m 38s
FreeBSD-x86_64 with rust stable	56s
Linux-x86_64 with rust stable	1m 15s
Linux-x86_64 (rustls-native-roots) with rust stable	1m 6s
Linux-x86_64 (native-tls) with rust stable	1m 6s
Linux-x86_64 (native-tls-vendored) with rust stable	2m 35s
Linux-aarch64 with rust stable	1m 2s
Linux-arm with rust stable	1m 15s
Linux-i686 with rust stable	50s
Linux-powerpc with rust stable	38s
Linux-powerpc64 with rust stable	38s
Linux-powerpc64le with rust stable	39s
Linux-riscv64 with rust stable	1m 5s
Linux-s390x with rust stable	57s
NetBSD-x86_64 with rust stable	1m 48s
Windows-aarch64 with rust stable	4m 2s
Windows-i686 with rust stable	3m 39s
Windows-x86_64 with rust stable	4m 2s
macOS-x86_64 with rust stable	1m 40s
macOS-aarch64 with rust stable	1m 43s
Linux-x86_64 with rust beta	1m 35s
Linux-x86_64 with rust nightly	1m 13s
	36m 22s

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