setup: fix spacing in error output messages from makefile

setup: fix spacing in error output messages from makefile
Looking over Brian's shoulder the other day, I saw a message that said "Please seedocs/install.html".
Looking at the source code of the Makefile, it seems like it should say

       @echo "ERROR: Not all of Tahoe's dependencies are in place.  Please see \
docs/install.html for help on installing dependencies."

instead of its current:

       @echo "ERROR: Not all of Tahoe's dependencies are in place.  Please see\
docs/install.html for help on installing dependencies."

But, I remember changing this more than once in the past, so either there is a different version of make somewhere which interprets trailing backslashes differently, or someone (possibly me) has repeatedly gotten confused about this issue.  Anyway, this time around, I'm trying:

       @echo "ERROR: Not all of Tahoe's dependencies are in place.  Please see docs/install.html for help on installing dependencies."

Even though it is > 80 chars.