Fix function name typo at the correct place.

As nandahkrishna writes in
https://github.com/Homebrew/brew/pull/11678#issuecomment-876356490,
completions/fish/brew.fish is generated automatically. This commit makes
the change in the proper template instead.

Library/Homebrew/completions/fish.erb

@@ -162,7 +162,7 @@ function __fish_brew_suggest_commands -d "Lists all commands names, including al
     end
 end
 
-function __fish_brew_suggest_diagnostic_check -d "List available diagnostic checks"
+function __fish_brew_suggest_diagnostic_checks -d "List available diagnostic checks"
     brew doctor --list-checks
 end
 

completions/fish/brew.fish

@@ -149,7 +149,7 @@ function __fish_brew_suggest_commands -d "Lists all commands names, including al
     end
 end
 
-function __fish_brew_suggest_diagnostic_checks -d "List available diagnostic checks"
+function __fish_brew_suggest_diagnostic_check -d "List available diagnostic checks"
     brew doctor --list-checks
 end