Fix grammar (#832)

* Update unit-testing.mdx

Fixes a grammatical error in the test annotation comment to improve clarity and correctness. The updated comment accurately states that the test is expected to raise an error.

* Update docs: grammar fix unit-testing.mdx

Fixes a grammatical error in the test annotation comment to improve clarity and correctness. The updated comment accurately states that the test is expected to raise an error.

Author: aptosnoob

apps/nextra/pages/en/build/smart-contracts/book/unit-testing.mdx

@@ -27,7 +27,7 @@ module 0x42::example {
 ### Expected Failure
 
 A test can also be annotated as an `#[expected_failure]`. This
-annotation marks that the test should is expected to raise an error.
+annotation marks that the test is expected to raise an error.
 
 You can ensure that a test is aborting with a specific abort `<code>`
 by annotating it with `#[expected_failure(abort_code = <code>)]`,