doc: add example for conversion from slice expressions to array ptr

Fixes #47599

Change-Id: I8f4ccd3b0c2bcdb057ee853163b4421229141333
Reviewed-on: https://go-review.googlesource.com/c/go/+/340351
Trust: Cuong Manh Le <[email protected]>
Run-TryBot: Cuong Manh Le <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: cuonglm

doc/go_spec.html

@@ -4329,6 +4329,7 @@ <h4 id="Conversions_from_slice_to_array_pointer">Conversions from slice to array
 <pre>
 s := make([]byte, 2, 4)
 s0 := (*[0]byte)(s)      // s0 != nil
+s1 := (*[1]byte)(s[1:])  // &amp;s1[0] == &amp;s[1]
 s2 := (*[2]byte)(s)      // &amp;s2[0] == &amp;s[0]
 s4 := (*[4]byte)(s)      // panics: len([4]byte) > len(s)