Fix typo in README.md call of get_cansim_table_overview

[lchski]

Calling as written in README threw an error:

Error in cansim_old_to_new(t) : 
  Unable to match old CANSIM table number 0000014
In addition: Warning message:
In cleaned_ndm_table_number(cansimTableNumber) :
  The cansim table number -14 used in this query is numeric,
it is safer to encode table numbers as character strings.

[mountainMath]

Thanks for this!