Update space_age_test.cpp, as they currently do not run on the website. - #1

In file included from /usr/local/include/catch2/benchmark/catch_benchmark.hpp:14,
                 from /usr/local/include/catch2/benchmark/catch_benchmark_all.hpp:24,
                 from /usr/local/include/catch2/catch_all.hpp:25,
                 from /tmp/space-age/space_age_test.cpp:3:
/tmp/space-age/space_age_test.cpp: In function 'void CATCH2_INTERNAL_TEST_2()':
/tmp/space-age/space_age_test.cpp:27:31: error: 'Approx' was not declared in this scope
   27 |     REQUIRE(age.on_earth() == Approx(31.69).margin(accuracy));
      |                               ^~~~~~
/tmp/space-age/space_age_test.cpp:27:31: note: suggested alternatives:
In file included from /usr/local/include/catch2/catch_all.hpp:26:
/usr/local/include/catch2/catch_approx.hpp:17:11: note:   'Catch::Approx'
   17 |     class Approx {
      |           ^~~~~~