go/token: fix grammar for precedence consts

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/8662045

src/pkg/go/token/token.go

@@ -243,8 +243,8 @@ func (tok Token) String() string {
 // A set of constants for precedence-based expression parsing.
 // Non-operators have lowest precedence, followed by operators
 // starting with precedence 1 up to unary operators. The highest
-// precedence corresponds serves as "catch-all" precedence for
-// selector, indexing, and other operator and delimiter tokens.
+// precedence serves as "catch-all" precedence for selector,
+// indexing, and other operator and delimiter tokens.
 //
 const (
 	LowestPrec  = 0 // non-operators