fix broken initial sync ref (#173)

Author: jeff-allen-mongo

source/core/replica-set-sync.txt

@@ -73,7 +73,7 @@ File Copy Based Initial Sync
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 :ref:`File copy based initial sync 
-<replica-set-initial-sync-filed-copy-based>` runs the initial sync
+<replica-set-initial-sync-file-copy-based>` runs the initial sync
 process by copying and moving files on the file system. 
 
 File copy based initial sync can be faster than