Turn the size limit reported in exception into float.

src/cgi.cpp

@@ -76,7 +76,7 @@ entry_type parse_formdata()
             throw runtime_error{format("The screenshot “{0:s}” is too big. "
                                        "The limit is {1:.1f} MiB.",
                                        screenshot.getFilename(),
-                                       size_limit / 1024 / 1024)};
+                                       size_limit / 1024.0 / 1024)};
         }
 
         const string filepath{files::get_tmpdir() / screenshot.getFilename()};