spec: fix grammar issue

Change-Id: If4d3b3965762c8979d304a82493c9eb1068ee13c
Reviewed-on: https://go-review.googlesource.com/c/go/+/642037
Reviewed-by: Robert Griesemer <[email protected]>
TryBot-Bypass: Robert Griesemer <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>

Author: alexandear

doc/go_spec.html

@@ -8514,7 +8514,7 @@ <h3 id="Package_unsafe">Package <code>unsafe</code></h3>
 <p>
 The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code>
 of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code>
-as if <code>v</code> was declared via <code>var v = x</code>.
+as if <code>v</code> were declared via <code>var v = x</code>.
 </p>
 <p>
 The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a>