docs: somewhat include f-strings in bnf grammar

docs/grammar.bnf

@@ -54,7 +54,7 @@
 
         DIGIT="regexp:\d"
         ALPHA="regexp:[a-zA-Z]"
-        STRING="regexp:('[^']*'|\"[^\"]*\")"
+        STRING="regexp:[fF]?('[^']*'|\"[^\"]*\")"
     ]
 }