Fix indentation in activate.fish (#2700)

Author: junzh0u

src/virtualenv/activation/fish/activate.fish

@@ -62,7 +62,7 @@ set -gx VIRTUAL_ENV '__VIRTUAL_ENV__'
 
 # https://github.com/fish-shell/fish-shell/issues/436 altered PATH handling
 if test (echo $FISH_VERSION | head -c 1) -lt 3
- set -gx _OLD_VIRTUAL_PATH (_bashify_path $PATH)
+  set -gx _OLD_VIRTUAL_PATH (_bashify_path $PATH)
 else
  set -gx _OLD_VIRTUAL_PATH $PATH
 end