x/crypto: fix typos

The typos were found by misspell tool.

Change-Id: I120740f12f7ba48330749ebf84050a7b98e01016
Reviewed-on: https://go-review.googlesource.com/24725
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

sha3/sha3.go

@@ -42,7 +42,7 @@ type state struct {
 	storage [maxRate]byte
 
 	// Specific to SHA-3 and SHAKE.
-	fixedOutput bool            // whether this is a fixed-ouput-length instance
+	fixedOutput bool            // whether this is a fixed-output-length instance
 	outputLen   int             // the default output size in bytes
 	state       spongeDirection // whether the sponge is absorbing or squeezing
 }