doc: update number of supported instruction sets

Current  number was out-of-date since adding MIPS.

Change-Id: I565342a92de3893b75cdfb76fa39f7fdf15672da
Reviewed-on: https://go-review.googlesource.com/22952Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/install-source.html

@@ -33,7 +33,7 @@ compiler using the GCC back end, see
 </p>
 
 <p>
-The Go compilers support five instruction sets.
+The Go compilers support six instruction sets.
 There are important differences in the quality of the compilers for the different
 architectures.
 </p>