docs: Fix "Layered chart with Dual-Axis" (Method syntax)

Discovered while writing tests for #3659.
The attribute syntax version is what is displayed, but the method syntax one was not equivalent.

Results in no `titleColor` being applied and is silently ignored during validation.

https://altair-viz.github.io/gallery/layered_chart_with_dual_axis.html

tests/examples_methods_syntax/layered_chart_with_dual_axis.py

@@ -15,12 +15,12 @@
 )
 
 area = base.mark_area(opacity=0.3, color='#57A44C').encode(
-    alt.Y('average(temp_max)').title('Avg. Temperature (°C)', titleColor='#57A44C'),
+    alt.Y('average(temp_max)').axis(title='Avg. Temperature (°C)', titleColor='#57A44C'),
     alt.Y2('average(temp_min)')
 )
 
 line = base.mark_line(stroke='#5276A7', interpolate='monotone').encode(
-    alt.Y('average(precipitation)').title('Precipitation (inches)', titleColor='#5276A7')
+    alt.Y('average(precipitation)').axis(title='Precipitation (inches)', titleColor='#5276A7')
 )
 
 alt.layer(area, line).resolve_scale(