HBASE-22873: Fixed typo in block caching docs

[lins05]

JIRA: https://issues.apache.org/jira/browse/HBASE-22873

[Apache-HBase]

🎊 +1 overall

Vote	Subsystem	Runtime	Comment
0	reexec	65	Docker mode activated.
		_ Prechecks _
+1	dupname	0	No case conflicting files found.
+1	@author	0	The patch does not contain any @author tags.
		_ master Compile Tests _
+1	mvninstall	344	master passed
0	refguide	361	branch has no errors when building the reference guide. See footer for rendered docs, which you should manually inspect.
		_ Patch Compile Tests _
+1	mvninstall	327	the patch passed
+1	whitespace	0	The patch has no whitespace issues.
0	refguide	339	patch has no errors when building the reference guide. See footer for rendered docs, which you should manually inspect.
		_ Other Tests _
+1	asflicense	18	The patch does not generate ASF License warnings.
		1528

Subsystem	Report/Notes
Docker	Client=19.03.1 Server=19.03.1 base: https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-506/1/artifact/out/Dockerfile
GITHUB PR	#506
JIRA Issue	HBASE-22873
Optional Tests	dupname asflicense refguide
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Build tool	maven
Personality	/home/jenkins/jenkins-slave/workspace/HBase-PreCommit-GitHub-PR_PR-506/out/precommit/personality/provided.sh
git revision	master / 43a0ec8
refguide	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-506/1/artifact/out/branch-site/book.html
refguide	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-506/1/artifact/out/patch-site/book.html
Max. process+thread count	96 (vs. ulimit of 10000)
modules	C: . U: .
Console output	https://builds.apache.org/job/HBase-PreCommit-GitHub-PR/job/PR-506/1/console
versions	git=2.11.0 maven=2018-06-17T18:33:14Z)
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