doc: replace -? with -h for bench_bitcoin help

The question mark (`?`) is interpreted as a wildcard for any single character in Zsh (see https://www.techrepublic.com/article/globbing-wildcard-characters-with-zsh), so `bench_bitcoin -?` will not work on systems using Zsh, such as macOS.

Since `-h` provides equivalent help functionality (as defined in https://github.com/bitcoin/bitcoin/blob/master/src/common/args.cpp#L684-L693), the `benchmarking.md` documentation has been updated to ensure compatibility with macOS.
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Author: l0rinc

doc/benchmarking.md

@@ -40,7 +40,7 @@ The output will look similar to:
 Help
 ---------------------
 
-    build/src/bench/bench_bitcoin -?
+    build/src/bench/bench_bitcoin -h
 
 To print the various options, like listing the benchmarks without running them
 or using a regex filter to only run certain benchmarks.