It’s not Kendall Square. Not yet. But two years after Mayor Thomas M. Menino branded a swath of South Boston’s waterfront as the Innovation District - a bid to lure big Web companies and young startups alike - a healthy crop of technology firms are moving in. Its office space is inexpensive, at least for Boston, and the neighborhood is catching on; any company looking for space in the city is looking there, commercial real estate brokers say.