Fix typographical errors

[Marcofann]

Description

This pull request addresses several typographical errors across multiple files in the repository. Specifically, it corrects the usage of indefinite articles ("a" vs. "an") and other minor grammatical issues for improved readability and accuracy. Below are the key changes:

1. File: chain/chain/src/orphan.rs

   • Fixed: "a orphan block" → "an orphan block"

2. File: core/async/README.md

   • Fixed: "a event-loop-based" → "an event-loop-based"

3. File: core/primitives/src/stateless_validation/stored_chunk_state_transition_data.rs

   • Fixed: "a implicit missing chunk" → "an implicit missing chunk"

4. File: runtime/near-vm-runner/src/instrument/stack_height/thunk.rs

   • Fixed: "a index of a thunk" → "an index of a thunk"

5. File: runtime/near-vm/test-api/src/sys/externals/global.rs

   • Fixed: "a immutable global" → "an immutable global"

6. File: runtime/runtime-params-estimator/src/trie.rs

   • Fixed: "a implementation" → "an implementation"

7. File: tools/congestion-model/src/main.rs

   • Fixed: "an half hour ago" → "a half hour ago"