doc: update the doc as unsigned int are supported now. (#700)

Author: chengxilo

test_context.go

@@ -237,6 +237,7 @@ func (ctx ScenarioContext) AfterStep(fn func(st *Step, err error)) {
 //
 // The stepFunc may accept one or several arguments of type:
 // - int, int8, int16, int32, int64
+// - uint, uint8, uint16, uint32, uint64
 // - float32, float64
 // - string
 // - []byte