small fixes to stopwatch codesandbox (reactjs#4110)

* small fixes to stopwatch codesandbox

noticed that the explanation for the first stopwatch codesandbox mentions "update the time every 10 milliseconds" so updated the codesandbox to reflect that

also there's a small nuanced bug in the second stopwatch codesandbox where each call to `handleStart()` sets a new interval without checking if there's already one ongoing. 

Ie: If the user accidentally double clicks the start button, they set two intervals for updating `now` every 10ms and then intervalRef only retains the second interval ID. Thus, it's impossible to actually stop the timer because `handleStop()` will only clear the latest set interval while the original one will keep executing.

* Update referencing-values-with-refs.md

* Update referencing-values-with-refs.md

* Update referencing-values-with-refs.md

Co-authored-by: dan <dan.abramov@gmail.com>

beta/src/pages/learn/referencing-values-with-refs.md

@@ -99,9 +99,9 @@ export default function Stopwatch() {
     setNow(Date.now());
 
     setInterval(() => {
-      // Update the current time every 100ms.
+      // Update the current time every 10ms.
       setNow(Date.now());
-    }, 100);
+    }, 10);
   }
 
   let secondsPassed = 0;
@@ -137,6 +137,8 @@ export default function Stopwatch() {
   function handleStart() {
     setStartTime(Date.now());
     setNow(Date.now());
+
+    clearInterval(intervalRef.current);
     intervalRef.current = setInterval(() => {
       setNow(Date.now());
     }, 10);