docs: typo in custom-tools.mdx (#3152)

packages/web/src/content/docs/custom-tools.mdx

@@ -15,7 +15,7 @@ Tools are defined as **TypeScript** or **JavaScript** files.
 
 ### Location
 
-They can defined:
+They can be defined:
 
 - Locally by placing them in the `.opencode/tool/` directory of your project.
 - Or globally, by placing them in `~/.config/opencode/tool/`.