docs: Readme files accuracy fix (a2aproject#1186)

## Problem
There are a couple of readme file with spelling, capitalization fixes to
align with rest of documents.


## Solution
“a2a specification” -> “A2A specification”.
“Helloworld” -> “Hello World”.


## Changes
docs/topics/a2a-and-mcp.md
docs/tutorials/python/4-agent-executor.md
docs/tutorials/python/7-streaming-and-multiturn.md
specification/grpc/README.md

## Testing
✅ Documentation builds successfully with mkdocs
✅ Markdown linting passes
✅ All internal anchor links verified
✅ No broken cross-references

## Issue
Fixes a2aproject#1185

---------

Signed-off-by: Yuri Zhao <[email protected]>

Author: yzhao244

docs/tutorials/python/4-agent-executor.md

@@ -48,7 +48,7 @@ Let's look at `agent_executor.py`. It defines `HelloWorldAgentExecutor`.
         3. It enqueues this message onto the `event_queue`. The underlying `DefaultRequestHandler` will then process this queue to send the response(s) to the client. For a single message like this, it will result in a single response for `message/send` or a single event for `message/stream` before the stream closes.
 
     - **`cancel`**:
-        The Helloworld example's `cancel` method simply raises an exception, indicating that cancellation is not supported for this basic agent.
+        The Hello World example's `cancel` method simply raises an exception, indicating that cancellation is not supported for this basic agent.
 
         ```python { .no-copy }
         --8<-- "https://raw.githubusercontent.com/a2aproject/a2a-samples/refs/heads/main/samples/python/agents/helloworld/agent_executor.py:HelloWorldAgentExecutor_cancel"

docs/tutorials/python/7-streaming-and-multiturn.md

@@ -1,6 +1,6 @@
 # 7. Streaming & Multi-Turn Interactions (LangGraph Example)
 
-The Helloworld example demonstrates the basic mechanics of A2A. For more advanced features like robust streaming, task state management, and multi-turn conversations powered by an LLM, we'll turn to the LangGraph example located in [`a2a-samples/samples/python/agents/langgraph/`](https://github.com/a2aproject/a2a-samples/tree/main/samples/python/agents/langgraph).
+The Hello World example demonstrates the basic mechanics of A2A. For more advanced features like robust streaming, task state management, and multi-turn conversations powered by an LLM, we'll turn to the LangGraph example located in [`a2a-samples/samples/python/agents/langgraph/`](https://github.com/a2aproject/a2a-samples/tree/main/samples/python/agents/langgraph).
 
 This example features a "Currency Agent" that uses the Gemini model via LangChain and LangGraph to answer currency conversion questions.
 

specification/grpc/README.md

@@ -1,6 +1,6 @@
 # Protocol Buffer Definitions
 
-This folder contains the a2a specification in Protocol Buffer (protobuf) format
+This folder contains the A2A specification in Protocol Buffer (protobuf) format
 
 ## Prerequisites