update README

Author: andrjohns

doxygen/README.md

@@ -8,7 +8,7 @@ Required Libraries
 ------------------
 Stan Math depends on four libraries:
 
-- Boost (version 1.78.0): [Boost Home Page](https://www.boost.org)
+- Boost (version 1.81.0): [Boost Home Page](https://www.boost.org)
 - Eigen (version 3.3.9: [Eigen Home Page](https://eigen.tuxfamily.org/index.php?title=Main_Page)
 - SUNDIALS (version 6.1.1): [Sundials Home Page](https://computing.llnl.gov/projects/sundials)
 - Intel TBB (version 2020.3): [Intel TBB Home Page](https://www.threadingbuildingblocks.org)
@@ -73,7 +73,7 @@ for subsequent compilations.
 The standalone makefile ensures that all the required `-I` include
 statements are given to the compiler, and the necessary libraries are
 linked: `~/stan-dev/math` and `~/stan-dev/math/lib/eigen_3.3.9` and
-`~/stan-dev/math/lib/boost_1.78.0` and
+`~/stan-dev/math/lib/boost_1.81.0` and
 `~/stan-dev/math/lib/sundials_6.1.1/include` and
 `~/stan-dev/math/lib/tbb_2020.3/include`. The
 `~/stan-dev/math/lib/tbb` directory is created by the `math-libs`