docs: fix heading and contribution link (#192)

* fix: doc headings of offsets

* fix: contribution guide link in readme

README.md

@@ -98,7 +98,7 @@ $ ./gradlew clean
 
 Development of Firehose happens in the open on GitHub, and we are grateful to the community for contributing bugfixes and improvements. Read below to learn how you can take part in improving Firehose.
 
-Read our [contributing guide](docs/contribute/contribution.md) to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to Firehose.
+Read our [contributing guide](docs/docs/contribute/contribution.md) to learn about our development process, how to propose bugfixes and improvements, and how to build and test your changes to Firehose.
 
 To help you get your feet wet and get you familiar with our contribution process, we have a list of [good first issues](https://github.com/odpf/firehose/labels/good%20first%20issue) that contain bugs which have a relatively limited scope. This is a great place to get started.
 

docs/docs/concepts/offsets.md

@@ -12,8 +12,8 @@ To use offset manager:
   * `setCommittable(Object batch)` to set the committable flag to be true.
 * `getCommittableOffset()` returns the largest offset that can be committed.
 
-##Implementation
-###Data Structures
+## Implementation
+### Data Structures
 * OffsetNode: A combination of topic, partition and the offset.
 * toBeCommittableBatchOffsets: A map of batch-keys and a set of OffsetNodes.
 * sortedOffsets: A map of topic-partition to a sorted list of OffsetNode.