Fix typos in {tree,xpath}.c (errror)

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
diff --git a/xpath.c b/xpath.c
index 0ca8cfa..89eaba8 100644
--- a/xpath.c
+++ b/xpath.c
@@ -618,7 +618,7 @@
     "Encoding error\n",
     "Char out of XML range\n",
     "Invalid or incomplete context\n",
-    "Stack usage errror\n",
+    "Stack usage error\n",
     "Forbidden variable\n",
     "?? Unknown error ??\n"	/* Must be last in the list! */
 };