Fix pragma mark typo

Built and noticed this warning
```
external/SDWebImage/SDWebImage/Core/NSButton+WebCache.m:152:9: warning: unknown pragma ignored [-Wunknown-pragmas]
#pragma mar - Private
        ^
```

SDWebImage/Core/NSButton+WebCache.m

@@ -149,7 +149,7 @@ static NSString * const SDAlternateImageOperationKey = @"NSButtonAlternateImageO
     [self sd_cancelImageLoadOperationWithKey:SDAlternateImageOperationKey];
 }
 
-#pragma mar - Private
+#pragma mark - Private
 
 - (NSURL *)sd_currentImageURL {
     return objc_getAssociatedObject(self, @selector(sd_currentImageURL));