typo correction

index.html

@@ -83,7 +83,7 @@
 							class="btn btn-dark my-1 mx-2"
 							onclick="document.getElementById('frequency-textarea').value = countDigrams(document.getElementById('cipher-text').value)"
 						>
-							Count Diagrams (2 CHARs)
+							Count Digrams (2 CHARs)
 						</button>
 						<button
 							class="btn btn-dark my-1 mx-2"