Update README.md

updating the build target to be correct for isolating a test

Author: pearcec

README.md

@@ -96,7 +96,7 @@ $ make test
 The majority of tests in the provider are Acceptance Tests - which provisions real resources in Azure. It's possible to run the entire acceptance test suite by running `make testacc` - however it's likely you'll want to run a subset, which you can do using a prefix, by running:
 
 ```sh
-make testacc SERVICE='resource' TESTARGS='-run=TestAccAzureRMResourceGroup' TESTTIMEOUT='60m'
+make acctests SERVICE='resource' TESTARGS='-run=TestAccAzureRMResourceGroup' TESTTIMEOUT='60m'
 ```
 
 The following Environment Variables must be set in your shell prior to running acceptance tests: