Fix typo (#724)

Signed-off-by: Natalie Arellano <narellano@vmware.com>

docs/for-buildpack-authors/how-to/write-buildpacks/use-exec.d/index.html

@@ -903,7 +903,7 @@ <h2 id="key-points">Key Points:</h2>
 
 2. Script Behavior:
 * **Inputs**
-    * A third open file descriptor (in addition to stdin and stdout).  The third open file descriptor is inherited from the calling process.
+    * A third open file descriptor (in addition to stdout and stderr).  The third open file descriptor is inherited from the calling process.
 * **Outputs**
     * Valid TOML describing environment variables in the form of key=value pairs. These variables are added to the application's runtime environment. The content should be written to file descriptor 3 (see examples for how to do this).
     * Exit Code: The scripts should exit with a status code of `0` to indicate success. A non-zero exit code will indicate an error and prevent the application from launching.