Correct an error in a comment for RK45.

This comment was describing the algorithm and the comment was copied from XMDS-1 where it was also incorrect.

git-svn-id: svn+ssh://svn.code.sf.net/p/xmds/code/trunk/xpdeint@2890 41b19578-b77a-4e3b-9682-c8570d237623