crypto/tls: fix docstring of Config.ClientSessionCache

Closes golang#21519

Change-Id: I1247e9435de93aae7e4db2b6e8e5be1b010c296b
Reviewed-on: https://go-review.googlesource.com/56832
Reviewed-by: Avelino <t@avelino.xxx>
Reviewed-by: Adam Langley <agl@golang.org>

common.go

@@ -471,8 +471,8 @@ type Config struct {
 	// connections using that key are compromised.
 	SessionTicketKey [32]byte
 
-	// SessionCache is a cache of ClientSessionState entries for TLS session
-	// resumption.
+	// ClientSessionCache is a cache of ClientSessionState entries for TLS
+	// session resumption.
 	ClientSessionCache ClientSessionCache
 
 	// MinVersion contains the minimum SSL/TLS version that is acceptable.