Fix grammar to accept self params in function pointer types

`fn(&self)`, `fn(&'static mut self)` etc. are syntactically valid
function pointer types

Author: fmease

src/types/function-pointer.md

@@ -12,7 +12,8 @@ FunctionTypeQualifiers -> Safety? (`extern` Abi?)?
 BareFunctionReturnType -> `->` TypeNoBounds
 
 MaybeNamedFunctionParameters ->
-    MaybeNamedParam ( `,` MaybeNamedParam )* `,`?
+      SelfParam `,`?
+    | (SelfParam `,`)? MaybeNamedParam ( `,` MaybeNamedParam )* `,`?
 
 MaybeNamedParam ->
     OuterAttribute* ( ( IDENTIFIER | `_` ) `:` )? ( Type | `...` )