[WebProfilerBundle] Small bugfix in CSS: don't make links bold. 

If the CSS in your application has something like `a { font-weight: bold; }` to make links bold by default, they will also show up **bold** in the profilers toolbar, which doesn't look right. This patch explicitly sets the font-weight to 'normal', as it should be. 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -