Fix copy-paste mistake

That text was probably copied from `prepend` above and not properly edited.

README.md

@@ -1131,7 +1131,7 @@ end
 
 #### append
 
-  Append some text to a field. This only works for string fields. Accepts a hash of fields to prepend. The keys are the field names and the values are the prefix strings. An example would be to add " (copied version)" to your description field. Don't forget to add a leading space if you want it. Passing a hash will add each key value pair to the list of append directives. If you wish to empty the list of directives, you may pass the hash inside of an array like this `[{:contents => " (copied version)"}]`.
+  Append some text to a field. This only works for string fields. Accepts a hash of fields to append. The keys are the field names and the values are the prefix strings. An example would be to add " (copied version)" to your description field. Don't forget to add a leading space if you want it. Passing a hash will add each key value pair to the list of append directives. If you wish to empty the list of directives, you may pass the hash inside of an array like this `[{:contents => " (copied version)"}]`.
 
 #### set