🩹 fix git init --initial-branch string instead of bool (#959)

Just using the completions I realized that:

```nu
# Create a new git repository
export extern "git init" [
  --initial-branch(-b)                                # initial branch name
  #should be
  --initial-branch(-b): string                        # initial branch name
]
```

because initial branch takes a string, it not a boolean

custom-completions/git/git-completions.nu

@@ -537,7 +537,7 @@ export extern "git stash drop" [
 
 # Create a new git repository
 export extern "git init" [
-  --initial-branch(-b)                                # initial branch name
+  --initial-branch(-b): string                         # initial branch name
 ]
 
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