update documentation of getOptimizely() (#27)

fix small grammar and spelling issues

Author: wangjoshuah

android-sdk/src/main/java/com/optimizely/ab/android/sdk/OptimizelyManager.java

@@ -164,15 +164,15 @@ public void stop(@NonNull Context context) {
     }
 
     /**
-     * Gets a chanced Optimizely instance
+     * Gets a cached Optimizely instance
      *
      * If {@link #start(Activity, OptimizelyStartListener)} or {@link #start(Context, OptimizelyStartListener)}
      * has not been called yet the returned {@link AndroidOptimizely} instance will be a dummy instance
      * that logs warnings in order to prevent {@link NullPointerException}.
      *
      * If {@link #getOptimizely(Context, int)} was used the built {@link AndroidOptimizely} instance
      * will be updated.  Using {@link #start(Activity, OptimizelyStartListener)} or {@link #start(Context, OptimizelyStartListener)}
-     * will update the cache instance with new {@link AndroidOptimizely} built from a cached local
+     * will update the cached instance with a new {@link AndroidOptimizely} built from a cached local
      * datafile on disk or a remote datafile on the CDN.
      * @return the cached instance of {@link AndroidOptimizely}
      */