doc: update Ubuntu Bionic Debian patch to remove asciidoctor option

The option "--failure-level" has been added in Asciidoctor 1.5.7 and the
version in Ubuntu Bionic is 1.5.5.

tools/debian/patches/weechat_ubuntu_bionic.patch

@@ -60,3 +60,15 @@ index 428f11ce9..e88f167b8 100644
      libxml2-dev,
      libcurl4-gnutls-dev,
      libgcrypt20-dev,
+diff --git a/doc/CMakeLists.txt b/doc/CMakeLists.txt
+index e3e5f7a85..af39249e4 100644
+--- a/doc/CMakeLists.txt
++++ b/doc/CMakeLists.txt
+@@ -23,7 +23,6 @@ if(ENABLE_MAN OR ENABLE_DOC)
+   if(ASCIIDOCTOR_FOUND)
+     # common asciidoctor arguments
+     set(ASCIIDOCTOR_ARGS
+-      --failure-level=WARNING
+       -a experimental
+       -a reproducible
+       -a "prewrap!"