Remove 'cospi' from docs function list

Function cospi is mentioned in the docs but it does not exist on the source

Author: ElDigoXD

docs/build_api.py

@@ -168,7 +168,6 @@ def build_index_page(groups):
             "ceil",
             "cos",
             "cosh",
-            "cospi",
             "erf",
             "erfc",
             "erfcinv",