Examples: Clean up. (#30979)

Seems like the sampling modes should be 'flat first' and 'flat either' instead of 'first' and 'either'

Author: Methuselah96

examples/webgpu_centroid_sampling.html

@@ -158,8 +158,8 @@
 
 				};
 
-				const withFlatFirstShader = createShader( 'flat', 'first' );
-				const withFlatEitherShader = createShader( 'flat', 'either' );
+				const withFlatFirstShader = createShader( THREE.InterpolationSamplingType.FLAT, THREE.InterpolationSamplingMode.FLAT_FIRST );
+				const withFlatEitherShader = createShader( THREE.InterpolationSamplingType.FLAT, THREE.InterpolationSamplingMode.FLAT_EITHER );
 
 				const withSampleShader = Fn( () => {
 
@@ -287,4 +287,4 @@
 
 		</script>
 	</body>
-</html>
+</html>