CLEANUP: fix typos in comments in ebtree

This is mainly about misspells of the word "occurrence". The misspells
are only located in code comments.
diff --git a/ebtree/eb32tree.h b/ebtree/eb32tree.h
index f33eb86..08ff900 100644
--- a/ebtree/eb32tree.h
+++ b/ebtree/eb32tree.h
@@ -129,7 +129,7 @@
 }
 
 /*
- * Find the first occurence of a key in the tree <root>. If none can be
+ * Find the first occurrence of a key in the tree <root>. If none can be
  * found, return NULL.
  */
 static forceinline struct eb32_node *__eb32_lookup(struct eb_root *root, u32 x)
@@ -180,7 +180,7 @@
 }
 
 /*
- * Find the first occurence of a signed key in the tree <root>. If none can
+ * Find the first occurrence of a signed key in the tree <root>. If none can
  * be found, return NULL.
  */
 static forceinline struct eb32_node *__eb32i_lookup(struct eb_root *root, s32 x)