[valhalla-docs] RFR: Minor grammatical fixes

[Dan Heidinga]

Some minor grammatical changes and a adjusted wording to reflect that Q may be flattened but flatness isn't guaranteed.

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Commit messages:
 - Minor grammatical fixes

Changes: https://git.openjdk.java.net/valhalla-docs/pull/8/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla-docs&pr=8&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/valhalla-docs/pull/8.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla-docs pull/8/head:pull/8

PR: https://git.openjdk.java.net/valhalla-docs/pull/8