fix: Don't escape parameter default values

Issue-191: mkdocstrings/python#191

Author: viktorlashchuk

src/mkdocstrings_handlers/python/templates/material/_base/signature.html.jinja

@@ -106,7 +106,7 @@ Context:
               {%- include "expression"|get_template with context -%}
             {%- endwith -%}
           {%- else -%}
-            {{ parameter.default }}
+            {{ parameter.default|safe }}
           {%- endif -%}
         {%- endif -%}