[libc++][docs] Fix inline code style - use double backticks (llvm#108974)

This is an apparrent omission. Single backick (Markdown style) was used
instead of double backtick for inline code style.

Author: H-G-Hristov

libcxx/docs/Contributing.rst

@@ -168,7 +168,7 @@ sure you don't forget anything:
 
 - Did you add the relevant feature test macro(s) for your feature? Did you update the ``generate_feature_test_macro_components.py`` script with it?
 - Did you run the ``libcxx-generate-files`` target and verify its output?
-- If needed, did you add `_LIBCPP_PUSH_MACROS` and `_LIBCPP_POP_MACROS` to the relevant headers?
+- If needed, did you add ``_LIBCPP_PUSH_MACROS`` and ``_LIBCPP_POP_MACROS`` to the relevant headers?
 
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