Fix an error in stacktraces.rst (JuliaLang#16115)

Function `stacktrace`  returned 4 vectors not 3.

doc/manual/stacktraces.rst

@@ -219,7 +219,7 @@ A call to :func:`backtrace` returns a vector of ``Ptr{Void}``, which may then be
  [inlined code from REPL.jl:3] eval_user_input at REPL.jl:62
  [inlined code from REPL.jl:92] anonymous at task.jl:63
 
-Notice that the vector returned by :func:`backtrace` had 15 pointers, while the vector returned by :func:`stacktrace` only has 3. This is because, by default, :func:`stacktrace` removes any lower-level C functions from the stack. If you want to include stack frames from C calls, you can do it like this::
+Notice that the vector returned by :func:`backtrace` had 15 pointers, while the vector returned by :func:`stacktrace` only has 4. This is because, by default, :func:`stacktrace` removes any lower-level C functions from the stack. If you want to include stack frames from C calls, you can do it like this::
 
  julia> stacktrace(stack, true)
  15-element Array{StackFrame,1}: