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
diff --git a/build/apex/runtests.sh b/build/apex/runtests.sh
index 4bf2dc4..2d9d21b 100755
--- a/build/apex/runtests.sh
+++ b/build/apex/runtests.sh
@@ -57,8 +57,8 @@
 Build (optional) and run tests on Android Runtime APEX package (on host).
 
   -s, --skip-build    skip the build step
-  -l, --list-files    list the contents of the ext4 image using `find`
-  -t, --print-tree    list the contents of the ext4 image using `tree`
+  -l, --list-files    list the contents of the ext4 image (\`find\`-like style)
+  -t, --print-tree    list the contents of the ext4 image (\`tree\`-like style)
   -h, --help          display this help and exit
 
 EOF