string.c: suppress a warning

* string.c (rb_str_casecmp_p): [DOC] use Unicode escape form to
  get rid of warning C4819 by Microsoft Visual C++.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57154 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -3210,7 +3210,7 @@ rb_str_casecmp(VALUE str1, VALUE str2)
  *     "aBcDeF".casecmp?("abcdef")    #=> true
  *     "abcdef".casecmp?("abcdefg")   #=> false
  *     "abcdef".casecmp?("ABCDEF")    #=> true
- *     "äöü".casecmp?("ÄÖÜ")          #=> true
+ *     "\u{e4 f6 fc}".casecmp?("\u{c4 d6 dc}") #=> true
  */
 
 static VALUE