audit: add check for indefinite article

at the beginning of desc.

Closes Homebrew/homebrew#42404.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Library/Homebrew/cmd/audit.rb

@@ -363,6 +363,10 @@ class FormulaAuditor
     if desc =~ /([Cc]ommand ?line)/
       problem "Description should use \"command-line\" instead of \"#{$1}\""
     end
+
+    if desc =~ %r[^([Aa]n?)\s]
+      problem "Please remove the indefinite article \"#{$1}\" from the beginning of the description"
+    end
   end
 
   def audit_homepage