templates: rename `Last change' column in hgwebdir repository list. This patch changes column headers in the templates that previously said `Last change' to `Last modified'. Neither code nor functionality are changed other than that. For some time now, I have been annoyed by the fact the `Last change' column didn't list the age of the youngest changeset in the repository, or at least tip. It just occurred to me that this is because the wording is slightly misleading; what the column in fact lists is when the repository was last *modified*, that is, when changesets was last added or removed from it. The word `change' can be understood as referring to the changeset itself. Using `changed' would be ever so slightly less amigous. However, the standard nomenclature in this case is `modification date' and `Last modified', which is incidentally entirely unambigous. Hence, `Last modified' is the wording used.