RFR: Fix typo in README.md

[Per Minborg]

This PR removes a redundant `=` character.

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Commit messages:
 - Fix typo in README.md

Changes: https://git.openjdk.org/jextract/pull/78/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=78&range=00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/78.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/78/head:pull/78

PR: https://git.openjdk.org/jextract/pull/78