Fix grammatical typo in custom_federated_algorithms_1.ipynb.
Before: "the fundamental of operators [..]"
After:  "the fundamental set of operators [..]"
"participants. We will refer to such operators as *federated operators*, to\n",
"distinguish them from ordinary (local) operators in Python.\n",
"\n",
- "The TFF type system, and the fundamental of operations supported in the TFF's\n",
+ "The TFF type system, and the fundamental set of operations supported in the TFF's\n",
"language, thus deviates significantly from those in Python, necessitating the\n",
"use of a dedicated representation.\n",
"\n",
