Fix markov chain generation to base the next ngram on the previous one - not the first one

MarkovGrams/UnweightedMarkovChain.cs

@@ -64,6 +64,7 @@ namespace MarkovGrams
 				string nextNgram = nextNgrams.ElementAt(rand.Next(0, nextNgrams.Count));
 				// Add the last character from the n-gram to the string we're building
 				result += nextNgram[nextNgram.Length - 1];
+				lastNgram = nextNgram;
 			}
 
 			return result;