Update example code in Enumerator#chunk documentation [ci skip]

The code assumed that /usr/share/dict/words did not use mixed
case, and it does at least on a few operating systems.

From CryptoRAT (Luke Elliot)

Fixes [Bug #16957]

enum.c

@@ -3203,7 +3203,7 @@ chunk_i(RB_BLOCK_CALL_FUNC_ARGLIST(yielder, enumerator))
  *  The following example counts words for each initial letter.
  *
  *    open("/usr/share/dict/words", "r:iso-8859-1") { |f|
- *      f.chunk { |line| line.ord }.each { |ch, lines| p [ch.chr, lines.length] }
+ *      f.chunk { |line| line.upcase.ord }.each { |ch, lines| p [ch.chr, lines.length] }
  *    }
  *    #=> ["\n", 1]
  *    #   ["A", 1327]