python caveats: use > instead of >>

The `homebrew.pth` should be a file with a single line in it.
Also (at least on zsh) if the file does not exist, `>>` will
result in an error. The `>` works in both, bash and zsh and
creates the file with the `echo`ed contet.

Closes Homebrew/homebrew#28201.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/caveats.rb

@@ -61,7 +61,7 @@ class Caveats
     if !file_path.readable? || !file_path.read.include?(site_packages)
       s = "If you need Python to find the installed site-packages:\n"
       s += "  mkdir -p #{dir}\n" unless dir_path.exist?
-      s += "  echo '#{site_packages}' >> #{file}"
+      s += "  echo '#{site_packages}' > #{file}"
     end
   end