doc/registry-web-api: Adjust user-agent header

Author: Turbo87

src/doc/src/reference/registry-web-api.md

@@ -42,7 +42,7 @@ Cargo sets the following headers for all requests:
 
 - `Content-Type`: `application/json` (for requests with a body payload)
 - `Accept`: `application/json`
-- `User-Agent`: The Cargo version such as `cargo 1.32.0 (8610973aa
+- `User-Agent`: The Cargo version such as `cargo/1.32.0 (8610973aa
   2019-01-02)`. This may be modified by the user in a configuration value.
   Added in 1.29.