Doc: add a space #2731

Job	Run time
external-stores (stores/rocksstore-v2)	5m 59s
lint	6m 36s
cluster-benchmark (cluster_benchmark)	49s
examples (stable, raft-kv-memstore)	2m 10s
examples (stable, raft-kv-rocksdb)	8m 9s
examples (nightly, raft-kv-memstore)	2m 4s
openraft-feature-test (nightly)	49s
openraft-test-bench (nightly)	53s
openraft-test (stable, 0)	2m 54s
openraft-feature-test (nightly, serde)	51s
examples (nightly, raft-kv-rocksdb)	12m 32s
openraft-test (nightly, 30)	3m 11s
openraft-feature-test (nightly, single-term-leader)	55s
openraft-test (nightly, 0, single-term-leader)	3m 1s
stores (memstore)	44s
openraft-feature-test (nightly, single-term-leader,serde,singlethreaded)	49s
stores (rocksstore)	6m 14s
Build (nightly, bench,serde,bt,singlethreaded)	47s
tests-feature-test (nightly)	2m 44s
stores (rocksstore-compat07)	6m 1s
stores (sledstore)	54s
tests-feature-test (nightly, single-term-leader)	2m 39s
tests-feature-test (nightly, loosen-follower-log-revert)	2m 47s
	1h 14m 32s

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