clang-tidy: use '' for single character find

Found with performance-faster-string-find

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/commands/CmdInfo.cpp

@@ -500,7 +500,7 @@ int CmdInfo::execute (std::string& output)
           if (end != std::string::npos)
           {
             auto uda = var.first.substr (12, end - 12);
-            auto dot = uda.find (".");
+            auto dot = uda.find ('.');
             if (dot == std::string::npos)
             {
               // urgency.uda.<name>.coefficient