fix spelling

--- a/src/HOL/Library/Heap.thy	Wed Apr 09 05:31:04 2008 +0200
+++ b/src/HOL/Library/Heap.thy	Wed Apr 09 05:44:14 2008 +0200
@@ -134,7 +134,7 @@
 
 text {*
   References and arrays are developed in parallel,
-  but keeping them seperate makes some later proofs simpler.
+  but keeping them separate makes some later proofs simpler.
 *}
 
 subsubsection {* Primitive operations *}