docs: Add wrap for pip function in fishshell

Now pip function mimic command pip exactly
Add correct markdown ref for fish commands

README.md

@@ -25,15 +25,15 @@ alias pip='function _pip(){
 
 For fish users, run on fish shell:
 
-```sh
-function pip
+```fish
+function pip --wraps="pip"
     set command $argv[1]
     set -e argv[1]
     switch "$command"
         case 'search'
             pip_search $argv
         case '*'
-            command pip "$command" $argv
+            command pip $command $argv
     end
 end