Switch README code syntax highlighting to Objective-C

Switch the code tags to use Objective-C, not shell. This improves the readability of the README.

**Before**:
```shell
-(int)numberOfCellsForCollapseClick {
    return (int)newInt;
}
```

**After**:
```objc
-(int)numberOfCellsForCollapseClick {
    return (int)newInt;
}
```