RFR: Sentence about build number

[Jesper Wilhelmsson]

Added a sentence to mention build numbers.

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Commit messages:
 - Sentence about build number

Changes: https://git.openjdk.java.net/guide/pull/63/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=63&range=00
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/guide/pull/63.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/63/head:pull/63

PR: https://git.openjdk.java.net/guide/pull/63