Fix typo in afl_driver.cpp (#4109)

tests/thirdparty/Fuzzer/afl/afl_driver.cpp

@@ -71,7 +71,7 @@ statistics from the file. If that fails then the process will quit.
 #endif
 
 // Used to avoid repeating error checking boilerplate. If cond is false, a
-// fatal error has occured in the program. In this event print error_message
+// fatal error has occurred in the program. In this event print error_message
 // to stderr and abort(). Otherwise do nothing. Note that setting
 // AFL_DRIVER_STDERR_DUPLICATE_FILENAME may cause error_message to be appended
 // to the file as well, if the error occurs after the duplication is performed.