Update index.ts, typo in hunk -> chunk in system prompt for unified diff edit

"hunk" is used in this whole file for "chunk". But for function names and variables it is ok. When the hard coded system prompt to be sent for LLMs contains "hunk" for "chunk". Then better to correct this typo atleast in system prompt.

Author: Bhartendu-Kumar

src/core/diff/strategies/new-unified/index.ts

@@ -131,7 +131,7 @@ Generate a unified diff that can be cleanly applied to modify code files.
 
 3. Group related changes:
    - Keep related modifications in the same chunk
-   - Start new hunks for logically separate changes
+   - Start new chunks for logically separate changes
    - When modifying functions/methods, include the entire block
 
 ## Requirements: