Fix broken </tt> tag on the clustering docs

There is a missing '<' in the clustering documentation for the
--id option [1] which makes the output look like this:

    –id can be used with –sparse/tt>.

This patch adds the missing symbol making it properly display
as:

    –id can be used with –sparse.

NOTE: This is a single character change better viewed with the
`--word-diff` option.

http://docs.graphlab.org/clustering.html#clustering_kmeans_id