fix a typo in Tutorial_hello_world.txt

[CCXXXI]

Summary of Changes

Fix a typo in Tutorial_hello_world.txt:

cgal/Documentation/doc/Documentation/Tutorials/Tutorial_hello_world.txt

Lines 74 to 76 in 8718201

	If you must ensure that your numbers get interpreted at their full precision
	you can use a \cgal kernel that performs exact predicates and
	extract constructions.

extract -> exact

[lrineau]

Thanks!