Fix docs for EVP_EncryptUpdate and EVP_DecryptUpdate

Fixes #4775

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from #4815)

doc/crypto/EVP_EncryptInit.pod

@@ -47,14 +47,14 @@ EVP_chacha20, EVP_chacha20_poly1305 - EVP cipher routines
  int EVP_EncryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
          ENGINE *impl, unsigned char *key, unsigned char *iv);
  int EVP_EncryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
-         int *outl, unsigned char *in, int inl);
+         int *outl, const unsigned char *in, int inl);
  int EVP_EncryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *out,
          int *outl);
 
  int EVP_DecryptInit_ex(EVP_CIPHER_CTX *ctx, const EVP_CIPHER *type,
          ENGINE *impl, unsigned char *key, unsigned char *iv);
  int EVP_DecryptUpdate(EVP_CIPHER_CTX *ctx, unsigned char *out,
-         int *outl, unsigned char *in, int inl);
+         int *outl, const unsigned char *in, int inl);
  int EVP_DecryptFinal_ex(EVP_CIPHER_CTX *ctx, unsigned char *outm,
          int *outl);