Fix a typo which caused a typeerror on string formatting

[Thomasvdh92]

Fix a typo where the string formatting caused a typeerror when only providing a datefrom and dateto, without an order number. See #4219

[codecov]

Codecov Report

Merging #4274 (0bb9e3b) into master (204b9a7) will increase coverage by 0.07%.
Report is 8 commits behind head on master.
The diff coverage is 0.00%.

❗ Current head 0bb9e3b differs from pull request most recent head 7e7ff9b. Consider uploading reports for the commit 7e7ff9b to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4274      +/-   ##
==========================================
+ Coverage   88.28%   88.35%   +0.07%     
==========================================
  Files         293      293              
  Lines       16046    16048       +2     
==========================================
+ Hits        14166    14180      +14     
+ Misses       1880     1868      -12     

Files	Coverage Δ
src/oscar/apps/customer/forms.py	71.60% <0.00%> (ø)

... and 4 files with indirect coverage changes

[viggo-devries]

@Thomasvdh92 tests failing unfortunately.