Improve comment on setting data_checksum GUC.

There was an extra space there, and "fixed" wasn't very descriptive.

src/backend/access/transam/xlog.c

@@ -4894,7 +4894,7 @@ ReadControlFile(void)
 				 errhint("It looks like you need to recompile or initdb.")));
 #endif
 
-	/* Make the fixed  settings visible as GUC variables, too */
+	/* Make the initdb settings visible as GUC variables, too */
 	SetConfigOption("data_checksums", DataChecksumsEnabled() ? "yes" : "no",
 					PGC_INTERNAL, PGC_S_OVERRIDE);
 }