update readme

README.md

@@ -134,5 +134,5 @@ packages/l/libpng/xmake.lua generated!
 $ xmake l scripts/test.lua --shallow -vD zlib
 $ xmake l scripts/test.lua --shallow -vD -p iphoneos zlib
 $ xmake l scripts/test.lua --shallow -vD -k shared -m debug zlib
-$ xmake l scripts/test.lua --shallow -vD --vs_runtime=MD zlib
+$ xmake l scripts/test.lua --shallow -vD --runtimes=MD zlib
 ```

README_zh.md

@@ -129,5 +129,5 @@ packages/l/libpng/xmake.lua generated!
 $ xmake l scripts/test.lua --shallow -vD zlib
 $ xmake l scripts/test.lua --shallow -vD -p iphoneos zlib
 $ xmake l scripts/test.lua --shallow -vD -k shared -m debug zlib
-$ xmake l scripts/test.lua --shallow -vD --vs_runtime=MD zlib
+$ xmake l scripts/test.lua --shallow -vD --runtimes=MD zlib
 ```