RFR: 7903871: Typo bug in doc/building.md of 'Rebuilding faster'

[SendaoYan]

Hi all,
The document of `doc/building.md` at chapter `Rebuilding faster with build/make.sh` has a typo bug, which may cause misunderstand.
Anyone wants to skip download dependencies, the option `--skip-download` should passed to `make/build.sh` explicitly. Because the default valune of variable `SKIP_DOWNLOAD` in [build-common.sh](https://github.com/openjdk/jtreg/blob/master/make/build-support/build-common.sh#L155) is empty, unless execute `make/build.sh` with `--skip-download` option.
Only fix the document bug, no risk.

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Commit messages:
 - 7903871: Typo bug in doc/building.md of 'Rebuilding faster'

Changes: https://git.openjdk.org/jtreg/pull/232/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=232&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903871
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/232.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/232/head:pull/232

PR: https://git.openjdk.org/jtreg/pull/232