Memory & Threads...: Fix ImageJ.cfg writing

ImageJ 1.x tries to be clever when parsing the ImageJ.cfg file to find
out whether the memory was specified in megabytes or gigabytes. If there
was no specifier, however, it defaults to kilobytes.

Work around this by making sure to write out the 'm' specifier.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>