Fix benchmarking example in the main readme

This uses the same fibonacci implementation as is used in the
benchmark test.

Closes #2568

README.md

@@ -47,7 +47,7 @@ TEST_CASE( "Factorials are computed", "[factorial]" ) {
 #include <cstdint>
 
 uint64_t fibonacci(uint64_t number) {
-    return number < 2 ? 1 : fibonacci(number - 1) + fibonacci(number - 2);
+    return number < 2 ? number : fibonacci(number - 1) + fibonacci(number - 2);
 }
 
 TEST_CASE("Benchmark Fibonacci", "[!benchmark]") {