fix: remove duplicate pricing entry for s2-pro in documentation (#59)

Author: twangodev

developer-guide/models-pricing/pricing-and-rate-limits.mdx

@@ -54,7 +54,6 @@ TTS pricing is based on the size of input text, measured in millions of UTF-8 by
 |--------------|------------------------|
 | `s2-pro`     | $15.00 / M UTF-8 bytes |
 | `s1`         | $15.00 / M UTF-8 bytes |
-| `s2-pro`         | $15.00 / M UTF-8 bytes |
 
 <Info>
 1M UTF-8 bytes is approximately 180,000 English words, or about 12 hours of speech