[Content] Chapter 1/4 - Fix placement of sentence introducing model categories

The sentence "This list is far from comprehensive, and is just meant to highlight a few of the different kinds of Transformer models. Broadly, they can be grouped into three categories:" is currently placed before the list of example models (InstructGPT, Llama, Mistral, etc.).

However, this sentence logically belongs immediately before the list of categories that starts with "GPT-like (also called auto-regressive Transformer models)". Placing it there will improve the flow and make it clearer that the categories refer to the grouping of Transformer models.