Svn doc edit for 4.0 branch name

[Mike Stump]

On http://gcc.gnu.org/wiki/SvnMerge
I changed:

$ svn switch svn+ssh://gcc.gnu.org/svn/gcc/branches/gcc-4.0-branch

to:

$ svn switch svn+ssh://gcc.gnu.org/svn/gcc/branches/gcc-4_0-branch

:-(  Took me a while to figure out what was wrong.  :-(

The error message is, uhm, well, unfortunate.

If the doc was correct, someone will have to rename some things and  
switch it back.