Add correct unit to generations

From testing (and reading https://github.com/miracle2k/tarsnapper/blob/master/tarsnapper/config.py#L92) values in seconds need an `s` appending.

Author: pbowyer

README.md

@@ -179,7 +179,7 @@ The design goals for this were as follows:
   have actually run or will run, but try to match the given deltas as
   closely as possible.
 
-The generations are defined by a list of deltas. ``60`` means a minute,
+The generations are defined by a list of deltas. ``60s`` means a minute,
 ``12h`` is half a day, ``7d`` is a week. The number of backups in each
 generation is implied by it's and the parent generation's delta.
 
@@ -215,4 +215,4 @@ off for weeks at a time, using the deltas ``1d 7d 30d 360d 18000d``:
       dropbox-20130511-055537
       dropbox-20130312-064042
       dropbox-20120325-054505
-      dropbox-20110331-12174
+      dropbox-20110331-12174