Rename HB_UNICODE_GENERAL_CATEGORY_COMBINING_MARK to HB_UNICODE_GENERAL_CATEGORY_SPACING_MARK

Spacing_Mark is the current Unicode long-name for this property value.
The previous name was wrongly carried from glib.
diff --git a/src/hb-icu.cc b/src/hb-icu.cc
index 384b77d..7b85cd5 100644
--- a/src/hb-icu.cc
+++ b/src/hb-icu.cc
@@ -109,7 +109,7 @@
 
   case U_NON_SPACING_MARK:		return HB_UNICODE_GENERAL_CATEGORY_NON_SPACING_MARK;
   case U_ENCLOSING_MARK:		return HB_UNICODE_GENERAL_CATEGORY_ENCLOSING_MARK;
-  case U_COMBINING_SPACING_MARK:	return HB_UNICODE_GENERAL_CATEGORY_COMBINING_MARK;
+  case U_COMBINING_SPACING_MARK:	return HB_UNICODE_GENERAL_CATEGORY_SPACING_MARK;
 
   case U_DECIMAL_DIGIT_NUMBER:		return HB_UNICODE_GENERAL_CATEGORY_DECIMAL_NUMBER;
   case U_LETTER_NUMBER:			return HB_UNICODE_GENERAL_CATEGORY_LETTER_NUMBER;