docs: Radar seriesField字段修复为非必需 (#2615)

Author: Plortinus

docs/api/plots/radar.en.md

@@ -60,7 +60,7 @@ The radar map maps to the field corresponding to the radius, typically a continu
 
 #### seriesField
 
-<description>**required** _string_</description>
+<description>**optional** _string_</description>
 
 A field that groups a radar map, usually corresponding to a classification field (**Attention：require a string field**). By the value of this field, the radar map will be divided into groups, separated by color, and overlaid.
 

docs/api/plots/radar.zh.md

@@ -60,7 +60,7 @@ radarPlot.render();
 
 #### seriesField
 
-<description>**required** _string_</description>
+<description>**optional** _string_</description>
 
 对雷达图进行分组的字段，一般对应一个分类字段 (**注意：文本型字段**)。通过该字段的值，雷达图将会被分为多个组，通过颜色进行区分，并上下重叠。