doc: rename Unshare to Unshareflags in go1.7 release notes

Implementation changed in https://golang.org/cl/23612.

Updates #15810

Change-Id: I8fff9e3aa3e54162546bb9ec1cc2ebba2b6d9fed
Reviewed-on: https://go-review.googlesource.com/23614
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.7.html

@@ -1131,7 +1131,7 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
 <a href="/pkg/syscall/#SysProcAttr"><code>SysProcAttr</code></a> struct
 (as used in
 <a href="/pkg/os/exec/#Cmd"><code>os/exec.Cmd</code></a>'s <code>SysProcAttr</code> field)
-has a new <code>Unshare</code> field.
+has a new <code>Unshareflags</code> field.
 If the field is nonzero, the child process created by
 <a href="/pkg/syscall/#ForkExec"><code>ForkExec</code></a>
 (as used in <code>exec.Cmd</code>'s <code>Run</code> method)