Fix art/build/apex/runtests.sh's usage command.

Escape backquotes in the here document used in the script's usage
text.

Also make the usage text more accurate.

Test: art/build/apex/runtests.sh --help
Bug: 113373927
Change-Id: I12d78b9b4d1b55b5162f66825ad654bc018a9d7b
1 file changed