Compile error while processing fitb examples

[DaveParillo]

What Course are you in
AuthorGuide

Describe the bug
During a runestone build, several errors in fitb examples:

source/directives/fitb_examples/fitb-ex1.txt:6: ERROR: Undefined substitution referenced: "blank".
source/directives/fitb_examples/fitb-ex1.txt:6: ERROR: Undefined substitution referenced: "blank".
source/directives/fitb_examples/fitb-ex2.txt:5: ERROR: Undefined substitution referenced: "blank".
source/directives/fitb_examples/fitb-ex4.txt:4: ERROR: Undefined substitution referenced: "blank".
source/directives/fitb_examples/fitb-ex5.txt:3: ERROR: Undefined substitution referenced: "blank".

From PR f65c776

runestone --version
Runestone version 6.3.16

The FITB examples still work correctly, this is an "ERROR" that does not actually seem to produce any erroneous results or prevent the build from completing.