[Docs] [Fix] Set $NVM_DIR to ".nvm" instead of "nvm"

- Operating system and version: Mac OS X Mojave

- What happened? $NVM_DIR was set to nvm

- What did you expect to happen? $NVM_DIR should be set to .nvm
I added the following line from the README to my .zshrc:
```
export NVM_DIR="${XDG_CONFIG_HOME/:-$HOME/.}nvm"
[ -s "$NVM_DIR/nvm.sh" ] && \. "$NVM_DIR/nvm.sh" # This loads nvm
```

I couldn't figure out why nvm wasn't loading, until I realized that
`echo NVM_DIR="${XDG_CONFIG_HOME/:-$HOME/.}nvm"` outputs `NVM_DIR=nvm`.

Changing the line to `export NVM_DIR="${XDG_CONFIG_HOME/:-$HOME/}.nvm"` fixed the problem. This outputs `NVM_DIR=.nvm` as expected. I tested this and the same behavior holds true in Bash as well.

install.sh

@@ -7,11 +7,7 @@ nvm_has() {
 }
 
 nvm_default_install_dir() {
-  if [ -n "${XDG_CONFIG_HOME-}" ]; then
-    printf %s "${XDG_CONFIG_HOME}/nvm"
-  else
-    printf %s "${HOME}/.nvm"
-  fi
+  [ -z "${XDG_CONFIG_HOME-}" ] && printf %s "${HOME}/.nvm" || printf %s "${XDG_CONFIG_HOME}/nvm"
 }
 
 nvm_install_dir() {