[DOC] Fix exception class [ci skip]

ref https://github.com/rurema/doctree/pull/816

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61212 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

re.c

@@ -1325,14 +1325,14 @@ rb_backref_set_string(VALUE string, long pos, long len)
  *      r.fixed_encoding?                               #=> true
  *      r.encoding                                      #=> #<Encoding:UTF-8>
  *      r =~ "\u{6666} a"                               #=> 2
- *      r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> ArgumentError
+ *      r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> Encoding::CompatibilityError
  *      r =~ "abc".force_encoding("euc-jp")             #=> 0
  *
  *      r = /\u{6666}/
  *      r.fixed_encoding?                               #=> true
  *      r.encoding                                      #=> #<Encoding:UTF-8>
  *      r =~ "\u{6666} a"                               #=> 0
- *      r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> ArgumentError
+ *      r =~ "\xa1\xa2".force_encoding("euc-jp")        #=> Encoding::CompatibilityError
  *      r =~ "abc".force_encoding("euc-jp")             #=> nil
  */