change file name so it can be copied without name change to a more public place.

R=rsc
DELTA=1090  (545 added, 545 deleted, 0 changed)
OCL=26629
CL=26629

doc/mem.html → doc/go_mem.html

+
 <h1>The Go memory model</h1>
 
 <h2>Introduction</h2>