MAPREDUCE-7255: fix typo in MapReduce documentaion example

[sergeioff]

Fix for typo in MapReduce documentation example (https://issues.apache.org/jira/browse/MAPREDUCE-7255)

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Comment
+0 🆗	reexec	30m 32s	Docker mode activated.
		_ Prechecks _
+1 💚	dupname	0m 0s	No case conflicting files found.
+0 🆗	markdownlint	0m 0s	markdownlint was not available.
+1 💚	@author	0m 0s	The patch does not contain any @author tags.
		_ trunk Compile Tests _
+1 💚	mvninstall	20m 57s	trunk passed
+1 💚	mvnsite	0m 38s	trunk passed
+1 💚	shadedclient	35m 12s	branch has no errors when building and testing our client artifacts.
		_ Patch Compile Tests _
+1 💚	mvninstall	0m 31s	the patch passed
+1 💚	mvnsite	0m 30s	the patch passed
+1 💚	whitespace	0m 0s	The patch has no whitespace issues.
+1 💚	shadedclient	14m 36s	patch has no errors when building and testing our client artifacts.
		_ Other Tests _
+1 💚	asflicense	0m 26s	The patch does not generate ASF License warnings.
		83m 29s

Subsystem	Report/Notes
Docker	Client=19.03.5 Server=19.03.5 base: https://builds.apache.org/job/hadoop-multibranch/job/PR-1793/1/artifact/out/Dockerfile
GITHUB PR	#1793
JIRA Issue	MAPREDUCE-7255
Optional Tests	dupname asflicense mvnsite markdownlint
uname	Linux aa7ccd02622c 4.15.0-66-generic #75-Ubuntu SMP Tue Oct 1 05:24:09 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	personality/hadoop.sh
git revision	trunk / f8644fb
Max. process+thread count	306 (vs. ulimit of 5500)
modules	C: hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core U: hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core
Console output	https://builds.apache.org/job/hadoop-multibranch/job/PR-1793/1/console
versions	git=2.7.4 maven=3.3.9
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[aajisaka]

+1