meson: rename html_help target to htmlhelp

Reported-by: Peter Eisentraut <peter.eisentraut@enterprisedb.com>
Discussion: https://postgr.es/m/3fc3bb9b-f7f8-d442-35c1-ec82280c564a@enterprisedb.com

doc/src/sgml/meson.build

@@ -123,7 +123,7 @@ if xsltproc_bin.found()
   # build multi-page html docs as part of docs target
   docs += html
 
-  html_help = custom_target('html_help',
+  htmlhelp = custom_target('htmlhelp',
     input: ['stylesheet-hh.xsl', postgres_full_xml],
     output: 'htmlhelp',
     depfile: 'htmlhelp.d',
@@ -131,7 +131,7 @@ if xsltproc_bin.found()
     command: [xsltproc, '--path', '@OUTDIR@', '-o', '@OUTDIR@/', xsltproc_flags, '@INPUT@'],
     build_by_default: false,
   )
-  alldocs += html_help
+  alldocs += htmlhelp
 
 
   # single-page HTML