HADOOP-17746. compatibility table in directory_markers.md doesn't render right.

[iwasakims]

https://issues.apache.org/jira/browse/HADOOP-17746

Doxia can not recognize the table as table due to dashes (--) at the bottom.

[hadoop-yetus]

🎊 +1 overall

Vote	Subsystem	Runtime	Logfile	Comment
+0 🆗	reexec	0m 34s		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	29m 23s		trunk passed
+1 💚	mvnsite	0m 47s		trunk passed
+1 💚	shadedclient	43m 43s		branch has no errors when building and testing our client artifacts.
			_ Patch Compile Tests _
+1 💚	mvninstall	0m 35s		the patch passed
+1 💚	blanks	0m 0s		The patch has no blanks issues.
+1 💚	mvnsite	0m 36s		the patch passed
+1 💚	shadedclient	13m 51s		patch has no errors when building and testing our client artifacts.
			_ Other Tests _
+1 💚	asflicense	0m 35s		The patch does not generate ASF License warnings.
		61m 1s

Subsystem	Report/Notes
Docker	ClientAPI=1.41 ServerAPI=1.41 base: https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3116/1/artifact/out/Dockerfile
GITHUB PR	#3116
Optional Tests	dupname asflicense mvnsite codespell markdownlint
uname	Linux 027c5dd785cf 4.15.0-60-generic #67-Ubuntu SMP Thu Aug 22 16:55:30 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	dev-support/bin/hadoop.sh
git revision	trunk / 95b06a7
Max. process+thread count	628 (vs. ulimit of 5500)
modules	C: hadoop-tools/hadoop-aws U: hadoop-tools/hadoop-aws
Console output	https://ci-hadoop.apache.org/job/hadoop-multibranch/job/PR-3116/1/console
versions	git=2.25.1 maven=3.6.3
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[steveloughran]

sorry, missed that this PR was here. merging to branch-3.3+