Fix header links to API subsections (#1)

Author: edenman

README.md

@@ -21,11 +21,11 @@ https://firebase.google.com.
    1. [Unit Testing](#unit-testing)
    1. [Integration Testing](#integration-testing)
 1. [Proguarding](#proguarding)
-   1. [APIs used via reflection](#APIs-used-via-reflection)
+   1. [APIs used via reflection](#apis-used-via-reflection)
    1. [APIs intended for developer
-   consumption](#APIs-intended-for-developer-consumption)
+   consumption](#apis-intended-for-developer-consumption)
    1. [APIs intended for other Firebase
-   SDKs](#APIs-intended-for-other-firebase-sdks)
+   SDKs](#apis-intended-for-other-firebase-sdks)
 1. [Publishing](#publishing)
    1. [Dependencies](#dependencies)
    1. [Commands](#commands)