chore: Fix spelling in contributing

Merge https://github.com/google/adk-python/pull/3394

This PR corrects misspellings identified by the [check-spelling action](https://github.com/marketplace/actions/check-spelling)

Note: while I use tooling to identify errors, the tooling doesn't _actually_ provide the corrections, I'm picking them on my own. I'm a human, and I may make mistakes.

### Testing Plan

The misspellings have been reported at https://github.com/jsoref/adk-python/actions/runs/19056081305/attempts/1#summary-54426435973

The action reports that the changes in this PR would make it happy: https://github.com/jsoref/adk-python/actions/runs/19056081446/attempts/1#summary-54426436321

**Unit Tests:**

- [ ] I have added or updated unit tests for my change.
- [ ] All unit tests pass locally.

_Please include a summary of passed `pytest` results._

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

_Please provide instructions on how to manually test your changes, including any
necessary setup or configuration. Please provide logs or screenshots to help
reviewers better understand the fix._

### 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.
- [ ] 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.

### Additional context

- https://github.com/google/adk-python/pull/3382#issuecomment-3488654110

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/adk-python/pull/3394 from jsoref:spelling-contributing c3d5e342c4350f7cae9f8f0c6638b176f2e30e80
PiperOrigin-RevId: 828659867

contributing/samples/adk_agent_builder_assistant/tools/search_adk_source.py

@@ -46,7 +46,7 @@ async def search_adk_source(
     max_results: Maximum number of results to return (default: 20)
     context_lines: Number of context lines to include around matches (default:
       3)
-    case_sensitive: Whether search should be case sensitive (default: False)
+    case_sensitive: Whether search should be case-sensitive (default: False)
 
   Returns:
     Dict containing search results:

contributing/samples/adk_agent_builder_assistant/tools/write_config_files.py

@@ -66,7 +66,7 @@ async def write_config_files(
 
   Args:
     configs: Dict mapping file_path to config_content (YAML as string)
-    backup_existing: Whether to create timest   amped backup of existing files
+    backup_existing: Whether to create timestamped backup of existing files
       before overwriting (default: False, User should decide)
     create_directories: Whether to create parent directories if they don't exist
       (default: True)