chore: Make README code blocks more easily copy pastable (#1420)

# What does this PR do?
When going through READMEs, I found that I had to keep editing the code
blocks since they were prefixed with `$ `. A common pattern is to triple
click (highlight all) a block and then copy paste. This minor change
will make this easier for folks to follow the READMEs.

[//]: # (If resolving an issue, uncomment and update the line below)
[//]: # (Closes #[issue-number])

## Test Plan
N/A

[//]: # (## Documentation)

llama_stack/distribution/ui/README.md

@@ -17,7 +17,7 @@ llama stack run together
 2. (Optional) Register datasets and eval tasks as resources. If you want to run pre-configured evaluation flows (e.g. Evaluations (Generation + Scoring) Page).
 
 ```bash
-$ llama-stack-client datasets register \
+llama-stack-client datasets register \
 --dataset-id "mmlu" \
 --provider-id "huggingface" \
 --url "https://huggingface.co/datasets/llamastack/evals" \
@@ -26,7 +26,7 @@ $ llama-stack-client datasets register \
 ```
 
 ```bash
-$ llama-stack-client benchmarks register \
+llama-stack-client benchmarks register \
 --eval-task-id meta-reference-mmlu \
 --provider-id meta-reference \
 --dataset-id mmlu \

llama_stack/providers/tests/README.md

@@ -20,10 +20,10 @@ dependencies. Below is the full configuration:
 
 
 ```bash
-$ cd llama-stack
-$ uv sync --extra dev --extra test
-$ uv pip install -e .
-$ source .venv/bin/activate
+cd llama-stack
+uv sync --extra dev --extra test
+uv pip install -e .
+source .venv/bin/activate
 ```
 
 ## Common options