Fix typo in example on testing.md.

The example says that `foo` should correspond to `provider_contents` earlier in the text.

Closes bazelbuild#14274.

PiperOrigin-RevId: 412977913

Author: cfredric

site/docs/skylark/testing.md

@@ -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