Update AntiXssEncoder.xml for issue 10378

[JeffSchwandt]

Summary

Fixes Issue #10378

• Corrected examples where (Space) was incorrectly encoded as  
• Added headings to the example table where they were missing.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-security, @vcsjones

[JeffSchwandt]

@dotnet-policy-service agree company="FNF"

[learn-build-service-prod]

Learn Build status updates of commit 4c9b3d3:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Web.Security.AntiXss/AntiXssEncoder.xml	✅Succeeded	View

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