docs(defi-loan): missing ErrInvalidSigner (#4428)

* defi loan tutorial: add ErrInvalidSigner

* defi loan tutorial: remove ErrInvalidSigner and update

* update

---------

Co-authored-by: Julien Robert <[email protected]>

docs/versioned_docs/version-v28/02-guide/05-loan.md

@@ -31,6 +31,7 @@ Notice the `--no-module` flag, in the next step we make sure the `bank` dependen
 
 2. **Create a Module:**
 
+
 Create a new "loan" module that is based on the standard Cosmos SDK `bank` module.
 
 ```bash
@@ -384,7 +385,33 @@ func (k msgServer) LiquidateLoan(goCtx context.Context, msg *types.MsgLiquidateL
 }
 ```
 
-Add the custom errors `ErrWrongLoanState` and `ErrDeadline`:
+```go title="x/loan/keeper/msg_update_params.go"
+package keeper
+
+import (
+	"context"
+
+	errorsmod "cosmossdk.io/errors"
+	sdk "github.com/cosmos/cosmos-sdk/types"
+
+	"loan/x/loan/types"
+)
+
+func (k msgServer) UpdateParams(goCtx context.Context, req *types.MsgUpdateParams) (*types.MsgUpdateParamsResponse, error) {
+	if k.GetAuthority() != req.Authority {
+		return nil, errorsmod.Wrapf(types.ErrInvalidSigner, "invalid authority; expected %s, got %s", k.GetAuthority(), req.Authority)
+	}
+
+	ctx := sdk.UnwrapSDKContext(goCtx)
+	if err := k.SetParams(ctx, req.Params); err != nil {
+		return nil, err
+	}
+
+	return &types.MsgUpdateParamsResponse{}, nil
+}
+```
+
+Add the errors `ErrInvalidSigner`, `ErrWrongLoanState`  and `ErrDeadline`:
 
 ```go title="x/loan/types/errors.go"
 package types
@@ -394,6 +421,7 @@ import (
 )
 
 var (
+  ErrInvalidSigner = sdkerrors.Register(ModuleName, 1100, "expected gov account as only signer for proposal message")
 	ErrWrongLoanState = sdkerrors.Register(ModuleName, 2, "wrong loan state")
 	ErrDeadline       = sdkerrors.Register(ModuleName, 3, "deadline")
 )