GEOS-5531 Add missing newlines after <TileMatrixSetLink> in WMTS Capabil...

GEOS-5531

The WMTS GetCapabilities lacks newlines in the XML output for the ```<TileMatrixSetLink>```. This should fix that issue so that the capabilities don't look like

```
<TileMatrixSetLink>      <TileMatrixSet>EPSG:2163</TileMatrixSet>
    </TileMatrixSetLink>  </Layer>
```
but rather
```
    <TileMatrixSetLink>
      <TileMatrixSet>EPSG:2163</TileMatrixSet>
    </TileMatrixSetLink>
```