Bump RDoc (ruby#14747)

Author: st0012

gems/bundled_gems

@@ -39,7 +39,7 @@ ostruct             0.6.3   https://github.com/ruby/ostruct
 pstore              0.2.0   https://github.com/ruby/pstore
 benchmark           0.4.1   https://github.com/ruby/benchmark
 logger              1.7.0   https://github.com/ruby/logger
-rdoc                6.14.2  https://github.com/ruby/rdoc
+rdoc                6.15.0  https://github.com/ruby/rdoc ac2a6fbf62b584a8325a665a9e7b368388bc7df6
 win32ole            1.9.2   https://github.com/ruby/win32ole
 irb                 1.15.2  https://github.com/ruby/irb d43c3d764ae439706aa1b26a3ec299cc45eaed5b
 reline              0.6.2   https://github.com/ruby/reline