docs: fix typo in CONTRIBUTING

Author: phanak-sap

CONTRIBUTING.md

@@ -24,7 +24,7 @@
   already a test going through the statements you have modified, you are probably
   wrong because you either added something new and it should be tested or you fixed
   a bug which was not detected by the test and hence the test must be enhanced
-  (ideally, you fist fix the test to reproduce the bug and then you fix the bug).
+  (ideally, you first fix the test to reproduce the bug and then you fix the bug).
 
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