crypto/rsa: fix docs for PrecomputedValues.Qinv

Fixes #7507.

LGTM=agl
R=agl
CC=golang-codereviews
https://golang.org/cl/74090043

src/pkg/crypto/rsa/rsa.go

@@ -60,7 +60,7 @@ type PrivateKey struct {
 
 type PrecomputedValues struct {
 	Dp, Dq *big.Int // D mod (P-1) (or mod Q-1)
-	Qinv   *big.Int // Q^-1 mod Q
+	Qinv   *big.Int // Q^-1 mod P
 
 	// CRTValues is used for the 3rd and subsequent primes. Due to a
 	// historical accident, the CRT for the first two primes is handled