Remove surplus slash when using WEB_URL

For consistency with the rabbitmq-server Makefile, WEB_URL ends in a
slash.  So any uses should not add another slash.

Author: dpw

dist.sh

@@ -168,7 +168,7 @@ function src-dist {
     cp -r lib/nunit tmp/srcdist/lib/
     cp Local.props.example tmp/srcdist/
     cp README.in tmp/srcdist/README
-    links -dump $WEB_URL/build-dotnet-client.html >> tmp/srcdist/README
+    links -dump ${WEB_URL}build-dotnet-client.html >> tmp/srcdist/README
     cp-license-to tmp/srcdist/
 
     ### Zip tmp/srcdist making $NAME_VSN the root dir in the archive