polygerrit-ui/README: lint against currently checked out branch

Change-Id: I8924d990cff1e2511c8c8437d20e5226c0d8b373
diff --git a/polygerrit-ui/README.md b/polygerrit-ui/README.md
index 3f84a58..07eba02 100644
--- a/polygerrit-ui/README.md
+++ b/polygerrit-ui/README.md
@@ -201,7 +201,7 @@
 * To run the linter on all of your local changes:
 
 ```sh
-git diff --name-only master | xargs node_modules/eslint/bin/eslint.js --ext .html,.js
+git diff --name-only HEAD | xargs node_modules/eslint/bin/eslint.js --ext .html,.js
 ```
 
 We also use the `polylint` tool to lint use of Polymer. To install polylint,