Irrelevant debug block in ddoc'd example of `assumeSafeAppend`

[quickfur]

Quote from https://dlang.org/library/object/assume_safe_append.html:

int[] a = [1, 2, 3, 4];

// Without assumeSafeAppend. Appending relocates.
int[] b = a [0 .. 3];
b ~= 5;
assert(a.ptr != b.ptr);

debug(SENTINEL) {} else
{
    // With assumeSafeAppend. Appending overwrites.
    int[] c = a [0 .. 3];
    c.assumeSafeAppend() ~= 5;
    writeln(a.ptr); // c.ptr
}

The bit about debug(SENTINEL) {} else is irrelevant and shouldn't be included in the ddoc.