Update faq.md (#38600)

Changed reference to gc to fully qualified (GC.gc()), as running `gc()` in a fresh julia session will not work.

Author: aramirezreyes

doc/src/manual/faq.md

@@ -28,7 +28,7 @@ session (technically, in module `Main`), it is always present.
 If memory usage is your concern, you can always replace objects with ones that consume less memory.
  For example, if `A` is a gigabyte-sized array that you no longer need, you can free the memory
 with `A = nothing`.  The memory will be released the next time the garbage collector runs; you can force
-this to happen with [`gc()`](@ref Base.GC.gc). Moreover, an attempt to use `A` will likely result in an error, because most methods are not defined on type `Nothing`.
+this to happen with [`GC.gc()`](@ref Base.GC.gc). Moreover, an attempt to use `A` will likely result in an error, because most methods are not defined on type `Nothing`.
 
 ### How can I modify the declaration of a type in my session?