Change "client key" to "client id" (#3130)

In #3118 @ryneeverett mentioned that "key" suggests that this is a
secret, when in truth it's just a user identifier. So "ID" is a better
word for it than "key".

src/commands/CmdShow.cpp

@@ -193,7 +193,7 @@ int CmdShow::execute (std::string& output)
     " sugar"
     " summary.all.projects"
     " sync.local.server_dir"
-    " sync.server.client_key"
+    " sync.server.client_id"
     " sync.server.encryption_secret"
     " sync.server.origin"
     " tag.indicator"