Add missing English keybind text for MIRV action (#2289)

## Description:

This PR adds the missing string for the MIRV keybind action in the User
Settings menu.
<img width="383" height="137" alt="スクリーンショット 2025-10-25 16 03 21"
src="https://github.com/user-attachments/assets/1a436fff-76a3-428a-bf14-c836ef9faf40"
/>

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

aotumuri

src/client/UserSettingModal.ts

@@ -523,8 +523,8 @@ export class UserSettingModal extends LitElement {
 
       <setting-keybind
         action="buildMIRV"
-        label=${translateText("user_setting.build_MIRV")}
-        description=${translateText("user_setting.build_MIRV_desc")}
+        label=${translateText("user_setting.build_mirv")}
+        description=${translateText("user_setting.build_mirv_desc")}
         defaultKey="Digit0"
         .value=${this.keybinds["buildMIRV"]?.key ?? ""}
         @change=${this.handleKeybindChange}