Make primeRange start parameter optional

Fix broken documentation.

src/__tests__/primes.spec.ts

@@ -130,6 +130,10 @@ describe('Prime range generator', () => {
     expect(primeRange(0, 4)).toEqual([2, 3, 5, 7]);
   });
 
+  it('produces the first 4 primes (start is optional)', () => {
+    expect(primeRange(4)).toEqual([2, 3, 5, 7]);
+  });
+
   it('produces 5 primes starting from the 999th odd prime', () => {
     expect(primeRange(999, 999 + 5)).toEqual([7919, 7927, 7933, 7937, 7949]);
   });

src/primes.ts

@@ -206,12 +206,16 @@ export function primes(start: number, end?: number) {
 }
 
 /**
- * Obtain a range of odd primes starting at ordinal the given ordinal.
+ * Obtain a range of primes starting at the given ordinal. Prime 2 has ordinal 0.
  * @param start 1-based ordinal of the nth odd prime to start from, or zero to include prime two.
  * @param end Range end. `end - start` elements are returned.
  * @returns The primes in the range.
  */
-export function primeRange(start: number, end: number) {
+export function primeRange(start: number, end?: number) {
+  if (end === undefined) {
+    end = start;
+    start = 0;
+  }
   start = Math.round(Math.max(0, start));
   end = Math.round(end);
   if (end <= start) {