Adds a sed line that allows to build slime.info with makeinfo v. 5.X

With newer versions of makeinfo, current recipe does not work due to an error that was previously a warning.

Adding an itemx to the texi file, prevents this from happening.

Sources:

* [Original thread with the fix](https://forums.gentoo.org/viewtopic-t-959158.html?sid=b8422e97ae46ebb98e3d3a2f7c52bc28)
* [Discussion on texinfo](http://savannah.gnu.org/bugs/?38997)