Make anchor link checking case-insensitive

[bmorelli25]

Follow-up to #287.

When building ES:

:: fail: Documentation.Builder.Log[0] `es-native-connectors-enabling-API-keys-for-upgraded-connectors` does not exist in es-native-connectors.md. (/Users/brandon/Documents/GitHub/elasticsearch.md/docs/es-native-connectors.md:139)

However, it does exist:

#### Enabling API keys for upgraded connectors [es-native-connectors-enabling-API-keys-for-upgraded-connectors]

Even though the case is correct in both places (the link and the anchor) this still fails link checking.

[reakaleek]

Will be fixed by #341