doc: use serial comma in errors docs

Refs: nodejs/node#11321
Refs: nodejs/node#17384

PR-URL: nodejs/node#43242
Reviewed-By: Mohammed Keyvanzadeh <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Darshan Sen <[email protected]>

Author: tniessen

doc/api/errors.md

@@ -1726,7 +1726,7 @@ time.
 > Stability: 1 - Experimental
 
 The `--input-type` flag was used to attempt to execute a file. This flag can
-only be used with input via `--eval`, `--print` or `STDIN`.
+only be used with input via `--eval`, `--print`, or `STDIN`.
 
 <a id="ERR_INSPECTOR_ALREADY_ACTIVATED"></a>
 
@@ -1984,7 +1984,7 @@ performing another operation.
 
 ### `ERR_INVALID_SYNC_FORK_INPUT`
 
-A `Buffer`, `TypedArray`, `DataView` or `string` was provided as stdio input to
+A `Buffer`, `TypedArray`, `DataView`, or `string` was provided as stdio input to
 an asynchronous fork. See the documentation for the [`child_process`][] module
 for more information.