Update brew tap instructions

Hey Aru! 👋 

Seems it's just `brew tap` not `brew tap install`:
```
[:~] 130 $ brew tap install arusahni/git-req
Updating Homebrew...
==> Auto-updated Homebrew!
Updated 3 taps (homebrew/cask-versions, homebrew/core and homebrew/cask).
==> New Formulae
bpython             dstask              pdftilecut          ptpython
==> Updated Formulae
Updated 132 formulae.
==> New Casks
pibar                      pragli                     twobird
==> Updated Casks
Updated 114 casks.
==> Deleted Casks
deadbeef-devel

Error: Invalid tap name 'install'
[:~] $ brew tap arusahni/git-req
==> Tapping arusahni/git-req
Cloning into '/usr/local/Homebrew/Library/Taps/arusahni/homebrew-git-req'...
remote: Enumerating objects: 14, done.
remote: Counting objects: 100% (14/14), done.
remote: Compressing objects: 100% (11/11), done.
remote: Total 14 (delta 4), reused 8 (delta 2), pack-reused 0
Receiving objects: 100% (14/14), done.
Resolving deltas: 100% (4/4), done.
Tapped 1 formula (28 files, 30.2KB).
```

Author: dschep

README.md

@@ -34,7 +34,7 @@ Installation
 
 1. Install it:
     * Homebrew (MacOS)
-        1. `brew tap install arusahni/git-req`
+        1. `brew tap arusahni/git-req`
         2. `brew install git-req`
     * AUR (Arch Linux)
         1. `yay -Sy git-req`