the problem is the underscore in the url. it's being interpreted as an
indicator for a subscript, which requires math. wrap the url with
this will require a package.
if you don't with to have hyperlinks, the
url package will suffice.
if you do wish to have hyperlinks, then use the
url). since this url is quite long, you may wish to allow
it to be broken across lines; the
breaklinks option will enable this:
see the package documentation regarding the places where breaks are permitted.
url package also has a (more complicated) line-breaking mechanism
available. again, see the package documentation for details.
as reminded by @egreg,
[breaklinks] works only if the output is
to be converted by
dvips and further converted to
pdf by an additional tool.