doc/go1.5.html: refer to ppc64 as 64-bit PowerPC, not Power 64

Saying "Power 64" was wrong for reasons I don't remember.
(Those reasons are why we stopped using GOARCH=power64.)

Change-Id: Ifaac78d5733bfc780df01b1a66da766af0b17726
Reviewed-on: https://go-review.googlesource.com/13675Reviewed-by: Rob Pike <r@golang.org>

doc/asm.html

@@ -741,10 +741,10 @@ Addressing modes:
 
 </ul>
 
-<h3 id="ppc64">Power 64, a.k.a. ppc64</h3>
+<h3 id="ppc64">64-bit PowerPC, a.k.a. ppc64</h3>
 
 <p>
-The Power 64 port is in an experimental state.
+The 64-bit PowerPC port is in an experimental state.
 </p>
 
 <p>

doc/go1.5.html

@@ -236,8 +236,8 @@ is only supported using external linking.
 </p>
 
 <p>
-Also available as experiments are <code>ppc64</code> (IBM Power 64)
-and <code>ppc64le</code> (IBM Power 64, little-endian).
+Also available as experiments are <code>ppc64</code>
+and <code>ppc64le</code> (64-bit PowerPC, big- and little-endian).
 Both these ports support <code>cgo</code> but
 only with internal linking.
 </p>
@@ -952,7 +952,7 @@ also now supports decoding of DWARF line tables.
 
 <li>
 The <a href="/pkg/debug/elf/"><code>debug/elf</code></a>
-package now has support for the 64-bit Power architecture.
+package now has support for the 64-bit PowerPC architecture.
 </li>
 
 <li>