chore: fixing various typos

Merge https://github.com/google/adk-python/pull/4175

### Link to Issue or Description of Change

**1. Link to an existing issue (if applicable):**

N/A: just fixing typos discovered while reading the repo

**2. Or, if no issue exists, describe the change:**

No code change, just typo fixes: see commit diffs for all details

**Problem:**

Trying to improve overall repo quality

**Solution:**

Fixing typos as they get discovered

### Testing Plan

N/A

**Unit Tests:**

N/A

**Manual End-to-End (E2E) Tests:**

N/A

### Checklist

- [X] I have read the [CONTRIBUTING.md](https://github.com/google/adk-python/blob/main/CONTRIBUTING.md) document.
- [X] I have performed a self-review of my own code.
- [ ] I have commented my code, particularly in hard-to-understand areas.
- [ ] I have added tests that prove my fix is effective or that my feature works.
- [X] New and existing unit tests pass locally with my changes.
- [ ] I have manually tested my changes end-to-end.
- [ ] Any dependent changes have been merged and published in downstream modules.

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/4175 from didier-durand:fix-typos-c 16e93ed2d9bc153fa0332ab1ae39633fcc5056e9
PiperOrigin-RevId: 858751240

contributing/samples/mcp_stdio_notion_agent/README.md

@@ -1,6 +1,6 @@
 # Notion MCP Agent
 
-This is an agent that is using Notion MCP tool to call Notion API. And it demonstrate how to pass in the Notion API key.
+This is an agent that is using Notion MCP tool to call Notion API. And it demonstrates how to pass in the Notion API key.
 
 Follow below instruction to use it: