runfiles: Fix usage instructions (#985)

The canonical location for the runfiles library going forward will be
the rules_python repo, so users should load from it. Since repository
names are essentially dynamic with Bzlmod, they should not be used in
import statements.

Author: fmeum

python/runfiles/runfiles.py

@@ -21,12 +21,12 @@
       py_binary(
           name = "my_binary",
           ...
-          deps = ["@bazel_tools//tools/python/runfiles"],
+          deps = ["@rules_python//python/runfiles"],
       )
 
 2.  Import the runfiles library.
 
-      from bazel_tools.tools.python.runfiles import runfiles
+      from python.runfiles import runfiles
 
 3.  Create a Runfiles object and use rlocation to look up runfile paths: