repo-file: s/content_checksum/contents_checksum/

This is what we call it everywhere else, so just be consistent.
It also lines up with metadata_checksum better.

https://bugzilla.gnome.org/show_bug.cgi?id=707727

src/libostree/ostree-repo-file.c

@@ -357,7 +357,7 @@ ostree_repo_file_tree_set_metadata (OstreeRepoFile *self,
 }
 
 const char *
-ostree_repo_file_tree_get_content_checksum (OstreeRepoFile  *self)
+ostree_repo_file_tree_get_contents_checksum (OstreeRepoFile  *self)
 {
   return self->tree_contents_checksum;
 }