Set language to "lus" for Lushai

Hello!

## Pull Request overview
* Set the language tag for easier discoverability

## Details
Apologies if this is incorrect, but I believe Mizo has the `lus` ISO code (https://en.wikipedia.org/wiki/Mizo_language), short for Lushai. I believe if we set `language: lus`, then this model will show up under https://huggingface.co/models?library=sentence-transformers&language=lus&sort=trending when people filter for Lushai.

Congratulations on the release!

- Tom Aarsen

README.md

@@ -7,6 +7,7 @@ tags:
 base_model: robzchhangte/MizBERT
 pipeline_tag: sentence-similarity
 license: apache-2.0
+language: lus
 ---
 
 # MizoEmbed-1