Make usage of foo clearer (JuliaLang#22317)

Replaces the placeholder `foo` with the built-in `norm` to make sure the example works when typed into a julia program.

Thanks to Avik Sengupta for the text and for all the commenters on https://discourse.julialang.org/t/what-does-foo-in-the-parallel-computing-docs-refer-to/4182 for making this happen.

doc/src/manual/parallel-computing.md

@@ -258,11 +258,15 @@ snippet:
 
 ```julia-repl
 A = rand(10,10)
-remotecall_fetch(()->foo(A), 2)
+remotecall_fetch(()->norm(A), 2)
 ```
 
+In this case [`norm`](@ref) is a function that takes 2D array as a parameter, and MUST be defined in
+the remote process. You could use any function other than `norm` as long as it is defined in the remote
+process and accepts the appropriate parameter.
+
 Note that `A` is a global variable defined in the local workspace. Worker 2 does not have a variable called
-`A` under `Main`. The act of shipping the closure `()->foo(A)` to worker 2 results in `Main.A` being defined
+`A` under `Main`. The act of shipping the closure `()->norm(A)` to worker 2 results in `Main.A` being defined
 on 2. `Main.A` continues to exist on worker 2 even after the call `remotecall_fetch` returns. Remote calls
 with embedded global references (under `Main` module only) manage globals as follows:
 
@@ -276,9 +280,9 @@ with embedded global references (under `Main` module only) manage globals as fol
 
  ```julia
  A = rand(10,10)
- remotecall_fetch(()->foo(A), 2) # worker 2
+ remotecall_fetch(()->norm(A), 2) # worker 2
  A = rand(10,10)
- remotecall_fetch(()->foo(A), 3) # worker 3
+ remotecall_fetch(()->norm(A), 3) # worker 3
  A = nothing
  ```