Directory_Replacement: fix 'section title out of sequence' warnings
    
    Running make to generate the docs yields the following warnings:
    
    asciidoctor: WARNING: Directory_Replacement.adoc: line 14: section title out of sequence: expected level 1, got level 2
    asciidoctor: WARNING: Directory_Replacement.adoc: line 33: section title out of sequence: expected level 1, got level 2
    
    The top section header level was changed in 64fc56f ("Convert to
    semantic linebreaks.", 2019-02-21) but those which followed were not.
    
    Adjust the section header levels to avoid these warnings.