docs: fix typo

Author: koichik

doc/api/child_processes.markdown

@@ -141,8 +141,8 @@ spawning the process with an empty environment rather than using
 API.
 
 There is a deprecated option called `customFds` which allows one to specify
-specific file descriptors for the stdio of the child process. This API is
-was not portable to all platforms and therefore removed.
+specific file descriptors for the stdio of the child process. This API was
+not portable to all platforms and therefore removed.
 With `customFds` it was possible to hook up the new process' [stdin, stdout,
 stderr] to existing streams; `-1` meant that a new stream should be created.
 Use at your own risk.