tools/flex: fix PKG_CPE_ID

cpe:/a:westes:flex is the correct CPE ID for flex:
https://nvd.nist.gov/products/cpe/search/results?keyword=cpe:2.3🅰️westes:flex

Fixes: c61a239514 (add PKG_CPE_ID ids to package and tools)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

tools/flex/Makefile

@@ -7,7 +7,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=flex
-PKG_CPE_ID:=cpe:/a:flex_project:flex
+PKG_CPE_ID:=cpe:/a:westes:flex
 PKG_VERSION:=2.6.4
 
 PKG_SOURCE:=$(PKG_NAME)-$(PKG_VERSION).tar.gz