docs: fix README breadth-first search example

[dajiaohuang]

Summary

Fix the README's breadth-first search example and matching API overview to use findAllBfs(from:goalTest:), the method currently provided by Graph. Also replace String.characters.first with String.first, since characters is unavailable in the documented Swift 5-and-later environment.

At master 6c5fb8828d056de4c9f922662748589ede201eff, compiling the example reports that WeightedGraph<String, Int> has no member findAll. After correcting the method name, compilation also reports that characters is unavailable. The corrected snippet compiles and returns the expected matching route on a two-vertex fixture.

Validation

• Swift 6.0.3, Ubuntu 24.04: swift build --jobs 1 passes.
• External SwiftPM consumer with a local package dependency: the README search snippet fails before these changes and compiles/runs after them.
• Additional self-authored graphs (at most five vertices): directed/undirected BFS/DFS, weighted directed/undirected shortest paths, MST, Codable roundtrips, collection access, and cycle/DAG API smoke checks pass.
• git diff --check passes.

No library behavior changes. The complete upstream test suite and performance tests were not run; local fixtures were intentionally limited to tiny graphs. macOS/Xcode, CocoaPods, Carthage, and the sample macOS app were not validated here.