RFR: 6914: Fix windows check formatting script

[Jean-Philippe Bempel]

because mvn is itself a script we need to call with call command
to get control back to the main script

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Commit messages:
 - Fix windows check formatting script

Changes: https://git.openjdk.java.net/jmc/pull/114/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=114&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6914
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jmc/pull/114.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/114/head:pull/114

PR: https://git.openjdk.java.net/jmc/pull/114