Bump rdoc to pull in RBS integration

Bump rdoc from bc0baee7 to a39b0bb3, which includes
ruby/rdoc#1665 ("Display RBS type signatures in
documentation"). PR ruby#1665 sources type information from inline `#:`
annotations and `.rbs` files (including the RBS stdlib) and renders it
in HTML and RI output, with type names linking to their documentation
pages.

Author: claude

gems/bundled_gems

@@ -37,7 +37,7 @@ ostruct             0.6.3   https://github.com/ruby/ostruct
 pstore              0.2.1   https://github.com/ruby/pstore
 benchmark           0.5.0   https://github.com/ruby/benchmark
 logger              1.7.0   https://github.com/ruby/logger
-rdoc                7.2.0   https://github.com/ruby/rdoc bc0baee787609f2205e8f1103f3e1d36b923184a
+rdoc                7.2.0   https://github.com/ruby/rdoc a39b0bb35b362959526cec7fd0231510b0cff9da
 win32ole            1.9.3   https://github.com/ruby/win32ole
 irb                 1.17.0  https://github.com/ruby/irb cfd0b917d3feb01adb7d413b19faeb0309900599
 reline              0.6.3   https://github.com/ruby/reline