Typo fix in test/README.md.

Build target name should be 'test-art-host-gtest' instead of
'test-art-host-gtest'.

Test: ran `m test-art-host-gtest` and ensured it does the right thing.
Change-Id: I4f230de748750d4406b0aa93cc803676b408760c
diff --git a/test/README.md b/test/README.md
index e661068..f855329 100644
--- a/test/README.md
+++ b/test/README.md
@@ -72,7 +72,7 @@
 the same directory as the code they exercise). Their source files usually end
 with the suffix `_test.cc.`. The construction logic of these tests is
 implemented in ART's build system (`Android.bp` and `Android*.mk` files). On
-host, these gtests can be run by executing `m test-art-host-gtests`. On device,
+host, these gtests can be run by executing `m test-art-host-gtest`. On device,
 the recommended approach is to run these tests in a chroot environment (see
 `README.chroot.md` in this directory).