HADOOP-17564. Fix typo in UnixShellGuide.html

[ashutoshcipher]

Description of PR

Fix typo in UnixShellGuide.html

• JIRA: HADOOP-17564

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 39s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+0 🆗	codespell	0m 0s		codespell was not available.
+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	38m 47s		trunk passed
+1 💚	mvnsite	1m 26s		trunk passed
+1 💚	shadedclient	59m 16s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 57s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	1m 16s		the patch passed
+1 💚	shadedclient	20m 22s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 53s		The patch does not generate ASF License warnings.
		83m 49s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4195/1/artifact/out/Dockerfile
GITHUB PR	#4195
Optional Tests	dupname asflicense mvnsite codespell markdownlint
uname	Linux ee294a65f0f2 4.15.0-169-generic #177-Ubuntu SMP Thu Feb 3 10:50:38 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / a208d19
Max. process+thread count	692 (vs. ulimit of 5500)
modules	C: hadoop-common-project/hadoop-common U: hadoop-common-project/hadoop-common
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-4195/1/console
versions	git=2.25.1 maven=3.6.3
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[ashutoshcipher]

@tomscut - Thanks for the review.