ChangeLog: fix notation

* ChangeLog: fix notation, Kernel#String is an instance method but
  not a singleton method, and a typo.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48261 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
ruby.h: add mark to PRIsVALUE

* include/ruby/ruby.h (PRIsVALUE), vsnprintf.c (BSD_vfprintf): add
  RUBY_PRI_VALUE_MARK to reduce danger of accidental conflict with
  plain "%i".  binary incompatible with extension libraries using
  PRIsVALUE and built for 2.1 and earlier.  [EXPERIMENTAL]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48262 b2dd03c8-39d4-4d8f-98ff-823fe69b080e