Fix mem example config in README

[mikehaertl]

The official examples on eclipse.jdt.ls all use -Xmx1G (max 1GB):

• https://github.com/eclipse/eclipse.jdt.ls/blob/master/README.md#running-from-the-command-line
• https://github.com/eclipse/eclipse.jdt.ls/wiki/Running-the-JAVA-LS-server-from-the-command-line#running-the-java-ls-server

Not sure why this was changed to -Xms1G (min 1GB). Changing this reduced the memory footprint from > 1GB to ~350MB for me.