fix link to installJinjaCompat
fix link to installJinjaCompat

Target was `/api.html#installjinjacompat` resolving to `https://mozilla.github.io/api.html#installjinjacompat` instead of `https://mozilla.github.io/nunjucks/api.html#installjinjacompat`. This fix changes the target to a relative path, resolving to the correct `nunjucks/api.html` page.
oddbird/nunjucks,kevinschaul/nunjucks,oddbird/nunjucks,kevinschaul/nunjucks,mozilla/nunjucks,mozilla/nunjucks,devoidfury/nunjucks,kevinschaul/nunjucks,internalfx/nunjucks,internalfx/nunjucks,internalfx/nunjucks,devoidfury/nunjucks,devoidfury/nunjucks,oddbird/nunjucks,oddbird/nunjucks,mozilla/nunjucks,internalfx/nunjucks,kevinschaul/nunjucks