doc: Fix install path of man page (#90)

Man pages go in /usr/share/man

  http://www.pathname.com/fhs/pub/fhs-2.3.html#USRSHAREMANMANUALPAGES

doc/CMakeLists.txt

 if (OPENDHT_TOOLS)
-	INSTALL(FILES ${CMAKE_CURRENT_SOURCE_DIR}/dhtnode.1 DESTINATION ${CMAKE_INSTALL_PREFIX}/man/man1)
+	INSTALL(FILES ${CMAKE_CURRENT_SOURCE_DIR}/dhtnode.1 DESTINATION ${CMAKE_INSTALL_PREFIX}/share/man/man1)
 endif ()