doc: R13 is stack pointer on ARM, not 386

Fix typo.

TBR=rsc

Change-Id: I85d1b46744a9a4524e7949e452cdebb53afe0740
Reviewed-on: https://go-review.googlesource.com/11959Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.html

@@ -360,7 +360,7 @@ Perhaps more important is that on machines where
 for a numbered register,
 such as <code>R13</code> for the stack pointer and
 <code>R15</code> for the hardware program counter
-on x86,
+on ARM,
 a reference to such a register that does not include a symbol
 is now illegal.
 For example, <code>SP</code> and <code>4(SP)</code> are