RFR: 8231854: Change Mercurial to git in various README files

[Kevin Rushforth]

https://bugs.openjdk.java.net/browse/JDK-8231854

Follow-on fix to [JDK-8231590](https://bugs.openjdk.java.net/browse/JDK-8231590) -- PR #3 -- to change Mercurial or hg to git in a few README files where appropriate.

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

Commits:
 - bb9544ac: 8231854: Change Mercurial to git in various README files

Changes: https://git.openjdk.java.net/jfx/pull/7/files
 Webrev: https://webrevs.openjdk.java.net/jfx/7/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8231854
  Stats: 7 lines in 4 files changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/jfx/pull/7.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/7/head:pull/7

PR: https://git.openjdk.java.net/jfx/pull/7