Update 2012-08-13-nsincrementalstore.md

Fixed return type of executeRequest:withContext:error: if resultType is NSCountResultType from NSNumber to NSArray containing a NSNumber.

Source:
file:///Users/christian/Library/Developer/Shared/Documentation/DocSets/com.apple.adc.documentation.AppleOSX10_8.CoreReference.docset/Contents/Resources/Documents/index.html#documentation/CoreData/Reference/NSIncrementalStore_Class/Reference/NSIncrementalStore.html#//apple_ref/occ/instm/NSIncrementalStore/executeRequest:withContext:error: