cmd/tsidp: remove backticks in README in shell example

Fixes #15818

Change-Id: I7a6f4c7368fed74b865a63acdea4559c3d0a0d09
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tsidp/README.md

@@ -35,7 +35,7 @@
 
    ```bash
    docker run -d \
-     --name `tsidp` \
+     --name tsidp \
      -p 443:443 \
      -e TS_AUTHKEY=YOUR_TAILSCALE_AUTHKEY \
      -e TS_HOSTNAME=idp \