Fix typo in 3.7 release notes

For this sample comparing to type predicates:
```typescript
if (isString(str)) {
    return str.toUppercase();
}
```
I believe you meant to put the correct method `str.toUpperCase();`

pages/release notes/TypeScript 3.7.md

@@ -288,7 +288,7 @@ function isString(val: any): val is string {
 
 function yell(str: any) {
     if (isString(str)) {
-        return str.toUppercase();
+        return str.toUpperCase();
     }
     throw "Oops!";
 }