[libc++][doc] Fixes FeatureTestMacroTable.html.

`utils/generate_feature_test_macro_components.py` uses the wrong
indentation. `:name: feature-status-table :widths: auto` is rendered as
text instead of being used by Sphinx to render the table properly.

This fixes the identation in the souce and updates the generated output.

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D112251