proc.c: fix symbol proc mark
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51996 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
proc.c: fix symbol proc mark
[ruby-core:70961] [Bug #11560]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51996 b2dd03c8-39d4-4d8f-98ff-823fe69b080e