Update child_process.md

fixed typo

Author: joseph0007

doc/api/child_process.md

@@ -842,7 +842,7 @@ pipes between the parent and child. The value is one of the following:
    file, socket, or a pipe with the child process. The stream's underlying
    file descriptor is duplicated in the child process to the fd that
    corresponds to the index in the `stdio` array. The stream must have an
-   underlying descriptor (file streams do not until the `'open'` event has
+   underlying descriptor (file streams do not open until the `'open'` event has
    occurred).
 7. Positive integer: The integer value is interpreted as a file descriptor
    that is open in the parent process. It is shared with the child