Fix some flaws in 'courses/machine_learning'

**Description**

During doing the Qwiklabs 'Maintaining Consistency in Training with Repeatable Splitting' I found a confusing sentence about varying RMSE value, that has the verb missing.

I also found some minor flaws and decided to fix them.

**Proposed changes**

- Add missing verb `differs` to `machine_learning/.../repeatable_splitting.ipynb`
- Add missing parentheses in `introduction_to_tensorflow/../1_core_tensorflow.ipynb`

**Related**
PR: https://github.com/GoogleCloudPlatform/training-data-analyst/pull/1095