alint.5: escape bare _

scd / markdown uses a pair of '_' to emphasize text and complains when
it cannot find a matching _.

Escape the '_' that's not used for markup but as actual content.

alint.5.scd

@@ -278,7 +278,7 @@ Severity: Serious, Certainty: Certain
 
 ## _builddir-is-set [AL26]
 
-_builddir is set instead of builddir, which is an old variable from before builddir
+\_builddir is set instead of builddir, which is an old variable from before builddir
 existed as a concept understood by abuild
 
 Severity: Serious, Certainty: Certain