Fix comments about filenames in some programs

programs/pkey/pk_sign.c

@@ -129,7 +129,7 @@ int main( int argc, char *argv[] )
     }
 
     /*
-     * Write the signature into <filename>-sig.txt
+     * Write the signature into <filename>.sig
      */
     mbedtls_snprintf( filename, sizeof(filename), "%s.sig", argv[2] );
 

programs/pkey/pk_verify.c

@@ -86,7 +86,7 @@ int main( int argc, char *argv[] )
     }
 
     /*
-     * Extract the signature from the text file
+     * Extract the signature from the file
      */
     ret = 1;
     mbedtls_snprintf( filename, sizeof(filename), "%s.sig", argv[2] );

programs/pkey/rsa_sign_pss.c

@@ -140,7 +140,7 @@ int main( int argc, char *argv[] )
     }
 
     /*
-     * Write the signature into <filename>-sig.txt
+     * Write the signature into <filename>.sig
      */
     mbedtls_snprintf( filename, 512, "%s.sig", argv[2] );
 

programs/pkey/rsa_verify_pss.c

@@ -100,7 +100,7 @@ int main( int argc, char *argv[] )
     mbedtls_rsa_set_padding( mbedtls_pk_rsa( pk ), MBEDTLS_RSA_PKCS_V21, MBEDTLS_MD_SHA256 );
 
     /*
-     * Extract the RSA signature from the text file
+     * Extract the RSA signature from the file
      */
     ret = 1;
     mbedtls_snprintf( filename, 512, "%s.sig", argv[2] );