fix typo in README.md

Author: mqy

examples/mulmat-tune/README.md

@@ -6,7 +6,8 @@ the following steps:
 1. read this file first and browse the files in dirs bench-out and analyze.
 2. pull and checkout codes, build and evaluate `mulmat-tune`.
 3. optionally run `tests/test-mulmat-tune`.
-4. place a `mulmat-tune.tx` into the parent dir of `main`, evaluate main.
+4. place file `mulmat-tune.txt` (generated from `mulmat-tune`) into the parent
+   dir of `main`, evaluate main.
 5. dive deeply into code details.
 
 ## Background