crypto: fix non-sentence in documentation for Decrypter

Comment change only.

Change-Id: I2e32c2b34d5a5659ead6d6082b06e1b039bf1147
Reviewed-on: https://go-review.googlesource.com/11852Reviewed-by: Russ Cox <rsc@golang.org>

src/crypto/crypto.go

@@ -126,8 +126,8 @@ type SignerOpts interface {
 }
 
 // Decrypter is an interface for an opaque private key that can be used for
-// asymmetric decryption operations. For example, an RSA key kept in a hardware
-// module.
+// asymmetric decryption operations. An example would be an RSA key
+// kept in a hardware module.
 type Decrypter interface {
 	// Public returns the public key corresponding to the opaque,
 	// private key.