go/ssa: fix typo in package docs

In go.dev/cl/457436, a new MultiConvert instruction was added, but it
was documented in the package docs as GenericConvert (the name
originally used in the CL).

Change-Id: I576184b599431163818b5cb6acc55b2773ac9997
Reviewed-on: https://go-review.googlesource.com/c/tools/+/495263
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>

Author: mdempsky

go/ssa/doc.go

@@ -66,7 +66,6 @@
 //	*FieldAddr            ✔               ✔
 //	*FreeVar              ✔
 //	*Function             ✔                               ✔ (func)
-//	*GenericConvert       ✔               ✔
 //	*Global               ✔                               ✔ (var)
 //	*Go                                   ✔
 //	*If                                   ✔
@@ -80,6 +79,7 @@
 //	*MakeMap              ✔               ✔
 //	*MakeSlice            ✔               ✔
 //	*MapUpdate                            ✔
+//	*MultiConvert         ✔               ✔
 //	*NamedConst                                           ✔ (const)
 //	*Next                 ✔               ✔
 //	*Panic                                ✔