HADOOP-17506. Fix typo in BUILDING.txt

[GauthamBanasandra]

• Fixed a couple of typos in
  BUILDING.txt

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 38s		Docker mode activated.
			_ Prechecks _
+1 💚	dupname	0m 0s		No case conflicting files found.
+1 💚	@author	0m 0s		The patch does not contain any @author tags.
			_ trunk Compile Tests _
+1 💚	shadedclient	20m 53s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	whitespace	0m 0s		The patch has no whitespace issues.
+1 💚	shadedclient	14m 22s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 33s		The patch does not generate ASF License warnings.
		38m 10s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-2662/1/artifact/out/Dockerfile
GITHUB PR	#2662
Optional Tests	dupname asflicense
uname	Linux 19d30181f8bc 4.15.0-112-generic #113-Ubuntu SMP Thu Jul 9 23:41:39 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / ad483fd
Max. process+thread count	717 (vs. ulimit of 5500)
modules	C: . U: .
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-2662/1/console
versions	git=2.25.1 maven=3.6.3
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[iwasakims]

+1. Thanks, @GauthamBanasandra.