download: fix nvm instructions (#7434)

Author: ljharb

apps/site/snippets/en/download/nvm.bash

@@ -1,6 +1,9 @@
 # Download and install nvm:
 curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.1/install.sh | bash
 
+# activate nvm (in lieu of restarting the shell)
+\. "${HOME}/.nvm/nvm.sh"
+
 # Download and install Node.js:
 nvm install ${props.release.major}