Fix AppVeyor URL in README.md

My AppVeyor account doesn't rebuild cppcodec when new changes are being
pushed to master.

Use efidler's AppVeyor URL instead. As tplgy admin, he has kindly added
AppVeyor integration to the upstream project and thus, updates get pushed
to his account.

README.md

@@ -1,6 +1,6 @@
 # cppcodec
 
-[![Build Status](https://travis-ci.org/tplgy/cppcodec.png)](https://travis-ci.org/tplgy/cppcodec) [![Build status](https://ci.appveyor.com/api/projects/status/175l4gr9pc5mmp9s?svg=true)](https://ci.appveyor.com/project/jpetso/cppcodec)
+[![Build Status](https://travis-ci.org/tplgy/cppcodec.png)](https://travis-ci.org/tplgy/cppcodec) [![Build status](https://ci.appveyor.com/api/projects/status/github/tplgy/cppcodec?branch=master&svg=true)](https://ci.appveyor.com/project/efidler/cppcodec)
 
 Header-only C++11 library to encode/decode base64, base64url, base32, base32hex
 and hex (a.k.a. base16) as specified in RFC 4648, plus Crockford's base32.