script: add flush example (#14082)

pages/linux/script.md

@@ -17,4 +17,8 @@
 
 - Record timing information (data is outputted to `stderr`):
 
-`script -t 2> {{path/to/timingfile}}`
+`script -t 2> {{path/to/timing_file}}`
+
+- Write out data as soon as it happens:
+
+`script -f {{path/to/file}}`