Minor gramatical error in docs

[wimachtendink]

an->a

"
You use the System.Buffers.IMemoryOwner interface to explicitly manage the ownership of a buffer. IMemoryOwner supports both ownership models. The component that has an IMemoryOwner reference owns the buffer. The following example uses an IMemoryOwner instance to reflect the ownership of an Memory buffer.
"

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Document Details

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• ID: 4819512e-1044-1c57-0de1-447164220fd5
• Version Independent ID: d6cf8d26-fb2f-7138-f0c7-ab529a5b9cde
• Content: Memory<T> and Span<T> usage guidelines
• Content Source: docs/standard/memory-and-spans/memory-t-usage-guidelines.md
• Product: dotnet
• GitHub Login: @tdykstra
• Microsoft Alias: tdykstra

[tdykstra]

Thanks, this will be fixed by PR #18685