Prerequisites (mark completed items with an [x]):

• I have checked that my issue type is not listed here https://github.com/woocommerce/woocommerce/issues/new/choose
• My issue is not a security issue, support request, bug report, enhancement or feature request (Please use the link above if it is).

Issue Description:

The 'email me when stock reaches' field allows us to enter non numerical data, we should limit this to numerical only.

Acceptance criteria

• 'email me when stock reaches' field only accepts numerical input