doc: fix typo

Fixes #13780.

Change-Id: I629e2ba79b74d693e04c3747812c9a686cae5335
Reviewed-on: https://go-review.googlesource.com/18218Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go_spec.html

@@ -5330,7 +5330,7 @@ the "for" statement's block but the <code>goto</code> is not.
 
 <p>
 A "fallthrough" statement transfers control to the first statement of the
-next case clause in a <a href="#Expression_switches">expression "switch" statement</a>.
+next case clause in an <a href="#Expression_switches">expression "switch" statement</a>.
 It may be used only as the final non-empty statement in such a clause.
 </p>