Tweak single-word label regex to fix pending spec.

lib/gitlab/markdown/label_reference_filter.rb

@@ -27,7 +27,7 @@ module Gitlab
         ~(
           (?<label_id>\d+)   | # Integer-based label ID, or
           (?<label_name>
-            [^'"&\?,\s]+     | # String-based single-word label title
+            [A-Za-z0-9_-]+   | # String-based single-word label title
             ['"][^&\?,]+['"]   # String-based multi-word label surrounded in quotes
           )
         )

spec/lib/gitlab/markdown/label_reference_filter_spec.rb

@@ -84,7 +84,6 @@ module Gitlab::Markdown
       end
 
       it 'links with adjacent text' do
-        skip 'FIXME (rspeicher): This will fail, because a period and parentheses are both currently valid in label names.'
         doc = filter("Label (#{reference}.)")
         expect(doc.to_html).to match(%r(\(<a.+><span.+>#{label.name}</span></a>\.\)))
       end