bump-formula-pr: fix typo in spec existence check

Closes Homebrew/homebrew#50472.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/dev-cmd/bump-formula-pr.rb

@@ -25,7 +25,7 @@ module Homebrew
     else
       [:stable, formula.stable]
     end
-    odie "#{formula}: no #{requested_spec} specification found!" unless formula
+    odie "#{formula}: no #{requested_spec} specification found!" unless formula_spec
 
     hash_type, old_hash = if (checksum = formula_spec.checksum)
       [checksum.hash_type.to_s, checksum.hexdigest]