fix: increase subsection grades rounding precision (openedx#1397)

We used two decimal digits to match the experience from the edx-platform.
However, openedx/edx-platform#27788 increased
the precision to reduce the impact of double rounding.

(cherry picked from commit e4a0105)

src/course-home/data/__snapshots__/redux.test.js.snap

@@ -782,7 +782,7 @@ Object {
         "gradingPolicy": Object {
           "assignmentPolicies": Array [
             Object {
-              "averageGrade": "1.00",
+              "averageGrade": "1.0000",
               "numDroppable": 1,
               "shortLabel": "HW",
               "type": "Homework",

src/course-home/data/api.js

@@ -18,7 +18,7 @@ const calculateAssignmentTypeGrades = (points, assignmentWeight, numDroppable) =
     // Calculate the average grade for the assignment and round it. This rounding is not ideal and does not accurately
     // reflect what a learner's grade would be, however, we must have parity with the current grading behavior that
     // exists in edx-platform.
-    averageGrade = (points.reduce((a, b) => a + b, 0) / points.length).toFixed(2);
+    averageGrade = (points.reduce((a, b) => a + b, 0) / points.length).toFixed(4);
     weightedGrade = averageGrade * assignmentWeight;
   }
   return { averageGrade, weightedGrade };