LibCore: Change a west const to an east const

Just fixing a little typo I found to make sure that everything adheres
to the style guide :^)

Userland/Libraries/LibCore/ConfigFile.cpp

@@ -45,7 +45,7 @@ NonnullRefPtr<ConfigFile> ConfigFile::open(String const& filename, int fd)
     return adopt_ref(*new ConfigFile(filename, fd));
 }
 
-ConfigFile::ConfigFile(const String& filename, AllowWriting allow_altering)
+ConfigFile::ConfigFile(String const& filename, AllowWriting allow_altering)
     : m_file(File::construct(filename))
 {
     if (!m_file->open(allow_altering == AllowWriting::Yes ? OpenMode::ReadWrite : OpenMode::ReadOnly))