docs: fix missing ai-sdk-v5 npm tag by using pinned version in README (#397)

Replace non-existent @ai-sdk-v5 dist-tag with pinned version @1.5.4
in the legacy AI SDK v5 install instructions.

Closes #390

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: robert-j-y

.changeset/dark-badgers-lie.md

@@ -0,0 +1,2 @@
+---
+---

README.md

@@ -19,13 +19,13 @@ yarn add @openrouter/ai-sdk-provider
 
 ```bash
 # For pnpm
-pnpm add @openrouter/ai-sdk-provider@ai-sdk-v5
+pnpm add @openrouter/ai-sdk-provider@1.5.4
 
 # For npm
-npm install @openrouter/ai-sdk-provider@ai-sdk-v5
+npm install @openrouter/ai-sdk-provider@1.5.4
 
 # For yarn
-yarn add @openrouter/ai-sdk-provider@ai-sdk-v5
+yarn add @openrouter/ai-sdk-provider@1.5.4
 ```
 
 ## Provider Instance