Update indexes.txt

Author: rustagir

source/fundamentals/indexes.txt

@@ -124,7 +124,7 @@ Single field indexes holds a reference to a field within a
 collection's documents. 
 
 This index improves single field queries and sort performance, and 
-supports TLS indexes that automatically remove documents from a
+supports TTL indexes that automatically remove documents from a
 collection after a certain amount of time.
 
 .. note::