Call copy method to avoid a potential issue for an NSString property

Summary:
According to rnystrom 's comment on #125 , we leave only two small changes in this PR.
1. Call copy method to avoid a potential issue for an `NSString` property.
2. fix a typo.

close #125

- [x] All tests pass. Demo project builds and runs.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/CONTRIBUTING.md)
Closes #131

Differential Revision: D4095492

Pulled By: rnystrom

fbshipit-source-id: b162842a38850c826685efd0746022daabe9d2b8

Source/IGListDiffable.h

@@ -11,7 +11,7 @@
 
 /**
  The IGListDiffable protocol provides the base methods needed to compare the identity and equality of two objects using
- one of the IGKAlgorithm functions.
+ one of the IGListDiff functions.
  */
 @protocol IGListDiffable
 

Source/IGListSingleSectionController.m

@@ -48,7 +48,7 @@ - (instancetype)initWithNibName:(NSString *)nibName
     IGParameterAssert(configureBlock != nil);
     IGParameterAssert(sizeBlock != nil);
     if (self = [super init]) {
-        _nibName = nibName;
+        _nibName = [nibName copy];
         _bundle = bundle;
         _configureBlock = [configureBlock copy];
         _sizeBlock = [sizeBlock copy];