[valhalla-docs] RFR: 01-background.md: fix link in-defense-of-erasure md

[Federico Medina]

Udate link `in-defense-of-erasure.html` -> `in-defense-of-erasure`

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Commit messages:
 - 01-background.md: fix link md (#1)

Changes: https://git.openjdk.org/valhalla-docs/pull/12/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla-docs&pr=12&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/valhalla-docs/pull/12.diff
  Fetch: git fetch https://git.openjdk.org/valhalla-docs.git pull/12/head:pull/12

PR: https://git.openjdk.org/valhalla-docs/pull/12