pacman.8: Fix typo

Signed-off-by: Allan McRae <allan@archlinux.org>

doc/pacman.8.asciidoc

@@ -464,7 +464,7 @@ Database Options (apply to '-D')[[QO]]
 
 *\--asexplicit* <package>::
 	Mark a package as explicitly installed; in other words, set their install
-	reason to be explicitly installed. This is useful it you want to keep a
+	reason to be explicitly installed. This is useful if you want to keep a
 	package installed even when it was initially installed as a dependency
 	of another package.