RFR: 8306408: Fix the format of several tables in building.md

[Dingli Zhang]

Hi all,

Currently, the format of several tables in building.md looks a little 
problematic and will incorrectly display the split line. In contrast, the 
corresponding building.html will display the tables correctly.

This patch will only change building.md and the format of the tables 
will display correctly.

Please take a look and have some reviews. Thanks a lot.

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Commit messages:
 - Fix the format of several tables in building.md

Changes: https://git.openjdk.org/jdk/pull/13523/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13523&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306408
  Stats: 24 lines in 1 file changed: 0 ins; 0 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/13523.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13523/head:pull/13523

PR: https://git.openjdk.org/jdk/pull/13523