libstdc++: Fix more doxygen markup for group close commands

Similar to r11-8009 but for /* @} */ comments this time, which should
be /** @} */ for Doxygen to recognize them.

libstdc++-v3/ChangeLog:

	* include/bits/random.h: Fix doxygen group commands.
	* include/bits/regex_constants.h: Likewise.
	* include/tr1/random.h: Likewise.