[foreign-memaccess+abi] RFR: Fix minor typos in ffi document

[Maurizio Cimadamore]

In my previous PR, I have missed a couple of issues in the FFI document.

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Commit messages:
 - Fix minor typos in ffi document

Changes: https://git.openjdk.org/panama-foreign/pull/819/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=819&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/819.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/819/head:pull/819

PR: https://git.openjdk.org/panama-foreign/pull/819