Fix installation instructions: prevent symlink referent from being overwritten

This change prevents system damage when a multi-call binary is installed such as "/usr/local/bin/youtube-dl -> /usr/local/bin/firejail". This fixes issue #30427.

README.md

@@ -20,12 +20,12 @@ youtube-dl - download videos from youtube.com or other video platforms
 
 To install it right away for all UNIX users (Linux, macOS, etc.), type:
 
-    sudo curl -L https://yt-dl.org/downloads/latest/youtube-dl -o /usr/local/bin/youtube-dl
+    curl -L https://yt-dl.org/downloads/latest/youtube-dl -o /tmp/youtube-dl && sudo mv /tmp/youtube-dl /usr/local/bin/
     sudo chmod a+rx /usr/local/bin/youtube-dl
 
 If you do not have curl, you can alternatively use a recent wget:
 
-    sudo wget https://yt-dl.org/downloads/latest/youtube-dl -O /usr/local/bin/youtube-dl
+    wget https://yt-dl.org/downloads/latest/youtube-dl -O /tmp/youtube-dl && sudo mv /tmp/youtube-dl /usr/local/bin/
     sudo chmod a+rx /usr/local/bin/youtube-dl
 
 Windows users can [download an .exe file](https://yt-dl.org/latest/youtube-dl.exe) and place it in any location on their [PATH](https://en.wikipedia.org/wiki/PATH_%28variable%29) except for `%SYSTEMROOT%\System32` (e.g. **do not** put in `C:\Windows\System32`).