Fix typo in example on testing.md.
The example says that `foo` should correspond to `provider_contents` earlier in the text.
Closes
@@ -169,7 +169,7 @@ Then:
* its test rule type should be named `foo_test` (`provider_contents_test`)
-* the label of the target of this rule type should be `foo`
+* the label of the target of this rule type should be `foo_test`
(`provider_contents_test`)
* the implementation function for the testing rule should be named
