Update doc.go [sha3/keccak] (#49)

Author: Deeptiman

crypto/sha3/keccak/doc.go

@@ -7,6 +7,7 @@
 //
 // Specification: FIPS 202 (SHA-3 Standard: Permutation-Based Hash and Extendable-Output Functions)
 // - https://csrc.nist.gov/files/pubs/fips/202/final/docs/fips_202_draft.pdf
+// - https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.202.pdf
 //
 // This package implementation follows the Keccak specification directly and is suitable for SHA-3 and
 // SHAKE construction.