.zshrc: Don't quote PATHs

It screws up the interpolation of ~.

Signed-off-by: Alan Orth <alan.orth@gmail.com>

.zshrc

@@ -80,7 +80,7 @@ if [[ -d ~/.pyenv ]]; then
 fi
 
 # If a private bin directory exists, add it to PATH
-[[ -d ~/bin ]] && PATH="$PATH:~/bin"
+[[ -d ~/bin ]] && PATH=$PATH:~/bin
 
 export PATH
 export MANPATH