Fix typo in HttpClientHandler.xml

[IngweLand]

Fixed duplicated text

Summary

Describe your changes here.

Fixes #Issue_Number (if available)

[dotnet-policy-service[bot]]

Tagging subscribers to this area: @dotnet/ncl

[learn-build-service-prod[bot]]

Learn Build status updates of commit fc871b8:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System.Net.Http/HttpClientHandler.xml	:white_check_mark:Succeeded	View

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