Fix llvm.memset semantics description

The description was referring to a ``src`` parameter probably copied over from ``llvm.memcpy``

llvm/docs/LangRef.rst

@@ -13864,8 +13864,8 @@ the argument.
 If ``<len>`` is 0, it is no-op modulo the behavior of attributes attached to
 the arguments.
 If ``<len>`` is not a well-defined value, the behavior is undefined.
-If ``<len>`` is not zero, both ``<dest>`` and ``<src>`` should be well-defined,
-otherwise the behavior is undefined.
+If ``<len>`` is not zero, ``<dest>`` should be well-defined, otherwise the
+behavior is undefined.
 
 '``llvm.sqrt.*``' Intrinsic
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^