Update scala_toolchains() docstring

Removes the reference to `@rules_scala_toolchains`, since that's not
really a public interface detail.

Prompted by a ping from @gergelyfabian in a Bazel Slack workspace direct
message, asking about the docstring currently reflected on `master`.

Author: mbland

scala/toolchains.bzl

@@ -47,7 +47,7 @@ def scala_toolchains(
         jmh = False,
         twitter_scrooge = False,
         twitter_scrooge_deps = {}):
-    """Instantiates @rules_scala_toolchains and all its dependencies.
+    """Instantiates rules_scala toolchains and all their dependencies.
 
     Provides a unified interface to configuring `rules_scala` both directly in a
     `WORKSPACE` file and in a Bazel module extension.