s/useable/usable/

hash.c

@@ -6792,7 +6792,7 @@ static const rb_data_type_t env_data_type = {
  *
  *  ==== User-Defined +Hash+ Keys
  *
- *  To be useable as a +Hash+ key, objects must implement the methods <code>hash</code> and <code>eql?</code>.
+ *  To be usable as a +Hash+ key, objects must implement the methods <code>hash</code> and <code>eql?</code>.
  *  Note: this requirement does not apply if the +Hash+ uses #compare_by_identity since comparison will then
  *  rely on the keys' object id instead of <code>hash</code> and <code>eql?</code>.
  *