RFR: 6942: The README is showing the wrong syntax for the build script

[Felipe Moraes]

6942: The README is showing the wrong syntax for the build script

-------------

Commit messages:
 - Fix readme instructions

Changes: https://git.openjdk.java.net/jmc/pull/132/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=132&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6942
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/132.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/132/head:pull/132

PR: https://git.openjdk.java.net/jmc/pull/132