Fix typo in diff prompt (#2410)

src/core/diff/strategies/multi-search-replace.ts

@@ -110,7 +110,7 @@ Search/Replace content with multi edits:
 :start_line:1
 :end_line:2
 -------
-def calculate_sum(items):
+def calculate_total(items):
     sum = 0
 =======
 def calculate_sum(items):

src/core/prompts/__tests__/__snapshots__/system.test.ts.snap

@@ -4101,7 +4101,7 @@ Search/Replace content with multi edits:
 :start_line:1
 :end_line:2
 -------
-def calculate_sum(items):
+def calculate_total(items):
     sum = 0
 =======
 def calculate_sum(items):