doctor: fix PATH issues in comments

Closes Homebrew/homebrew#25170.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/doctor.rb

@@ -500,7 +500,7 @@ def check_user_path_2
     <<-EOS.undent
       Homebrew's bin was not found in your PATH.
       Consider setting the PATH for example like so
-          echo export PATH="#{HOMEBREW_PREFIX}/bin:$PATH" >> ~/.bash_profile
+          echo export PATH='#{HOMEBREW_PREFIX}/bin:$PATH' >> ~/.bash_profile
     EOS
   end
 end
@@ -514,7 +514,7 @@ def check_user_path_3
         Homebrew's sbin was not found in your PATH but you have installed
         formulae that put executables in #{HOMEBREW_PREFIX}/sbin.
         Consider setting the PATH for example like so
-            echo export PATH="#{HOMEBREW_PREFIX}/sbin:$PATH" >> ~/.bash_profile
+            echo export PATH='#{HOMEBREW_PREFIX}/sbin:$PATH' >> ~/.bash_profile
       EOS
     end
   end