Fix LinguaTagger ImportError referring to langdetect instead of lingua

[Chessing234]

Bug

LinguaTagger.__init__ checks LINGUA_AVAILABLE but raises ImportError("langdetect is not installed, ..."), pointing users to the wrong package.

https://github.com/allenai/dolma/blob/96f8b07/python/dolma/taggers/language.py#L254-L257

def __init__(self) -> None:
    super().__init__()
    if not LINGUA_AVAILABLE:
        raise ImportError("langdetect is not installed, please run `pip install dolma[lang]`.")
    self.detector = LanguageDetectorBuilder.from_languages(*Language.all()).build()

Root cause

Copy-paste from LangdetectTagger.__init__ at line 204-206, which correctly reports langdetect when LANGDETECT_AVAILABLE is false. The message in LinguaTagger was never updated for the new availability flag.

Fix

Change the message to "lingua is not installed, ..." so it names the actually-missing package. The pip install dolma[lang] hint is unchanged because the lang extras include lingua-language-detector.