Refine comment for DateTime function in speed.go

The comment for the DateTime function in the speed.go file was slightly confusing and inaccurate. It has been updated to more accurately reflect its purpose and functionality. It now clearly states that the function returns the DateTime when the Speed was checked, as originally intended.

speed.go

@@ -29,8 +29,7 @@ func (s Speed) IsAvailable() bool {
 	return !s.na
 }
 
-// DateTime returns true if an Speed value was
-// available at time of query
+// DateTime returns the DateTime when the Speed was checked
 func (s Speed) DateTime() time.Time {
 	return s.dt
 }