Change to optimize_for_target_gateset in docs (quantumlib#6658)

- This doc had an incorrect name for the function.
Changed to the correct one.

Author: dstrain115

cirq/transformers/target_gatesets/compilation_target_gateset.py

@@ -76,7 +76,7 @@ def transformer_with_kwargs(
 class CompilationTargetGateset(ops.Gateset, metaclass=abc.ABCMeta):
     """Abstract base class to create gatesets that can be used as targets for compilation.
 
-    An instance of this type can be passed to transformers like `cirq.convert_to_target_gateset`,
+    An instance of this type can be passed to transformers like `cirq.optimize_for_target_gateset`,
     which can transform any given circuit to contain gates accepted by this gateset.
     """