go.talks/pkg/present: fix a couple of vet-detected issues

R=adg, bradfitz
CC=golang-dev
https://golang.org/cl/10457043
diff --git a/pkg/present/code.go b/pkg/present/code.go
index 3098900..367e4b2 100644
--- a/pkg/present/code.go
+++ b/pkg/present/code.go
@@ -157,7 +157,7 @@
 	switch n := arg.(type) {
 	case int:
 		if n <= 0 || n > max {
-			return 0, "", false, fmt.Errorf("%%d is out of range", n)
+			return 0, "", false, fmt.Errorf("%d is out of range", n)
 		}
 		return n, "", true, nil
 	case string:
diff --git a/pkg/present/parse.go b/pkg/present/parse.go
index e43b91b..2e263c5 100644
--- a/pkg/present/parse.go
+++ b/pkg/present/parse.go
@@ -90,8 +90,8 @@
 
 func (s Section) Sections() (sections []Section) {
 	for _, e := range s.Elem {
-		if s, ok := e.(Section); ok {
-			sections = append(sections, s)
+		if section, ok := e.(Section); ok {
+			sections = append(sections, section)
 		}
 	}
 	return