Fix typo in configurations (#578)

Author: xufan6

configurations/use_skylib_build_setting/README.md

@@ -5,5 +5,5 @@ To test it out, cd into this directory and run the following:
 
 ```
 $ bazel build :ice-cream // => "flavor: strawberry"
-$ bazel build :ice-cream --//use_skylib_build_settings:flavor=rocky-road => "flavor: rocky-road"
+$ bazel build :ice-cream --//use_skylib_build_setting:flavor=rocky-road => "flavor: rocky-road"
 ```