README: fix coverity link

coverity link was pointing to the wrong project

Signed-off-by: Alejandro Mery <amery@geeks.cl>

README.md

 [![Travis Build Status](https://travis-ci.org/warmcat/libwebsockets.png)](https://travis-ci.org/warmcat/libwebsockets)
 [![Appveyor Build status](https://ci.appveyor.com/api/projects/status/baadj9gkpxhi5rhy)](https://ci.appveyor.com/project/warmcat/libwebsockets)
-[![Coverity Scan Build Status](https://scan.coverity.com/projects/2506/badge.svg)](https://scan.coverity.com/projects/2506)
+[![Coverity Scan Build Status](https://scan.coverity.com/projects/3576/badge.svg)](https://scan.coverity.com/projects/3576)
 
 libwebsockets
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