doc: fix incorrect example in asm.html

Fixes #13845.

Change-Id: Ie83179b2d20c47a0296645d9e2fdc43271be495a
Reviewed-on: https://go-review.googlesource.com/18307Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/asm.html

@@ -176,7 +176,7 @@ This form is used to name global functions and data.
 Adding <code>&lt;&gt;</code> to the name, as in <span style="white-space: nowrap"><code>foo&lt;&gt;(SB)</code></span>, makes the name
 visible only in the current source file, like a top-level <code>static</code> declaration in a C file.
 Adding an offset to the name refers to that offset from the symbol's address, so
-<code>a+4(SB)</code> is four bytes past the start of <code>foo</code>.
+<code>foo+4(SB)</code> is four bytes past the start of <code>foo</code>.
 </p>
 
 <p>