all: fix up language in a couple of comments

Leftovers from 11699043

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/12558046

src/pkg/go/ast/ast.go

@@ -528,7 +528,7 @@ func IsExported(name string) bool {
 }
 
 // IsExported reports whether id is an exported Go symbol
-// (i.e., whether it begins with an uppercase letter).
+// (that is, whether it begins with an uppercase letter).
 //
 func (id *Ident) IsExported() bool { return IsExported(id.Name) }
 

src/pkg/html/template/css.go

@@ -99,7 +99,7 @@ func decodeCSS(s []byte) []byte {
 	return b
 }
 
-// isHex reports reports whether the given character is a hex digit.
+// isHex reports whether the given character is a hex digit.
 func isHex(c byte) bool {
 	return '0' <= c && c <= '9' || 'a' <= c && c <= 'f' || 'A' <= c && c <= 'F'
 }