ShowReadme: fix name of README file
ShowReadme: fix name of README file

It changed to Markdown awhile ago, but was never changed in the
ShowReadme command to match.

Of course, ImageJ still displays the Markdown-formatted text in a plain
text editor, which is a little unfortunate, but it's better than
receiving an error message as happens currently.

TehSAUCE/imagej,TehSAUCE/imagej,biovoxxel/imagej,biovoxxel/imagej,biovoxxel/imagej,TehSAUCE/imagej