Kernel: Rename "descriptor" to "description" where appropriate.

Now that FileDescription is called that, variables of that type should not
be called "descriptor". This is kinda wordy but we'll get used to it.

Kernel/KSyms.cpp

@@ -157,8 +157,8 @@ void load_ksyms()
 {
     auto result = VFS::the().open("/kernel.map", 0, 0, VFS::the().root_custody());
     ASSERT(!result.is_error());
-    auto descriptor = result.value();
-    auto buffer = descriptor->read_entire_file();
+    auto description = result.value();
+    auto buffer = description->read_entire_file();
     ASSERT(buffer);
     load_ksyms_from_data(buffer);
 }