leveldb.cmake: fix comment: firestore.patch -> firestore.patch.txt

Author: dconeybe

cmake/external/leveldb.cmake

@@ -18,9 +18,9 @@ if(TARGET leveldb)
   return()
 endif()
 
-# This version must be kept in sync with the version in firestore.patch.
+# This version must be kept in sync with the version in firestore.patch.txt.
 # If this version ever changes then make sure to update the version in
-# firestore.patch accordingly.
+# firestore.patch.txt accordingly.
 set(version 1.23)
 
 ExternalProject_Add(