Pin zsh-async and pure to the main branch

For unclear reasons, zplug is not pointing at the main branch and is
probably pointing at master. Pinning to this branch moved the status
from failed to OK. Made change following reading this issues (because
Pure uses zsh-async): mafredri/zsh-async#61

zsh/zshrc

@@ -60,8 +60,8 @@ export ZPLUG_HOME=/usr/local/opt/zplug
 source $ZPLUG_HOME/init.zsh
 source $(brew --prefix nvm)/nvm.sh
 
-zplug 'mafredri/zsh-async'
-zplug 'sindresorhus/pure'
+zplug 'mafredri/zsh-async', from:"github", at:"main", use:"async.zsh"
+zplug 'sindresorhus/pure', use:pure.zsh, at:"main", from:github, as:theme
 zplug "zsh-users/zsh-completions"
 zplug "zsh-users/zsh-autosuggestions"
 zplug "zsh-users/zsh-syntax-highlighting"