docs: Fix appveyor download URL

The download URL should not point PR build. It should point the latest
build of master branch.

README.md

@@ -18,7 +18,7 @@ together.
 If you want to try the latest universal-ctags without building it yourself...
 
 ### Windows
-- Go to https://ci.appveyor.com/project/masatake/ctags
+- Go to https://ci.appveyor.com/project/masatake/ctags/branch/master
   - Click the ```compiler=msvc_msys2, ARCH=x64, ...``` (or ```compiler=msvc_msys2, ARCH=x86, ...```) build.
   - View the *Artifacts* tab and download ```ctags-XXXXXX-x64.zip``` (or ```ctags-XXXXXX-x86.zip```). (```XXXXXX``` is a version number or a commit ID.)
   - Add the binary folder to your PATH.