Fix potential leak based on analyzer advice

```
Analyzer Warnings:

  0) Pods/ObjectiveSugar/Classes/NSArray+ObjectiveSugar.m:172:12: Potential leak of an object stored into 'descriptor':
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169 
170 - (NSArray *)sortBy:(NSString*)key; {
171     NSSortDescriptor *descriptor = [[NSSortDescriptor alloc] initWithKey:key ascending:YES];
172     return [self sortedArrayUsingDescriptors:@[descriptor]];
        ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
173 }
174 <rest>

Pods/ObjectiveSugar/Classes/NSArray+ObjectiveSugar.m:171:36: Method returns an Objective-C object with a +1 retain count
Pods/ObjectiveSugar/Classes/NSArray+ObjectiveSugar.m:172:12: Object leaked: object allocated and stored into 'descriptor' is not referenced later in this execution path and has a retain count of +1
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** ANALYZE FAILED ** (13137 ms)
```