Fix formatting of warning message in meson.build

Author: tobiasdiez

src/doc/meson.build

@@ -2,7 +2,9 @@ if not get_option('build-docs')
   subdir_done()
 endif
 
-warning('Documentation building enabled, generating targets may be slow. To disable this, pass -Dbuild-docs=false.')
+warning(
+  'Documentation building enabled, generating targets may be slow. To disable this, pass -Dbuild-docs=false.',
+)
 
 sphinx_check = py_module.find_installation(required: false, modules: ['sphinx'])
 if not sphinx_check.found()