themes_customize.md: fix Window.border sample (IJSDK-923)
themes_customize.md: fix Window.border sample (IJSDK-923)

artspb/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs,JetBrains/intellij-sdk-docs,artspb/intellij-sdk-docs