Update ArrayBufferWriter`1.xml (dotnet#6627)

Fix typo in GetSpan<T> (looks like copy/paste error).

Author: dlech

xml/System.Buffers/ArrayBufferWriter`1.xml

@@ -296,7 +296,7 @@ This method never returns <xref:System.Memory%601.Empty?displayProperty=nameWith
  <returns>A span of at least <paramref name="sizeHint" /> in length. If <paramref name="sizeHint" /> is not provided or is equal to 0, some non-empty buffer is returned.</returns>
  <remarks>
  <format type="text/markdown"><![CDATA[
-This method never returns <xref:System.Memory%601.Empty?displayProperty=nameWithType>.
+This method never returns <xref:System.Span%601.Empty?displayProperty=nameWithType>.
  ]]></format>
  </remarks>
  <exception cref="T:System.ArgumentException">