Added a line about interactive usage with mathlib

Usually `bc` isn't very useful if it doesn't return have floating point number where appropriate. e.g., 3/4 returns 0 when started with `-i` but for most users they would expect 0.75 to be returned so the argument `-l`does what the user would generally want.

pages/common/bc.md

@@ -6,6 +6,10 @@
 
 `bc -i`
 
+- Run calculator in interactive mode with floating point support:
+
+`bc -l`
+
 - Calculate the result of an expression:
 
 `bc <<< "(1 + 2) * 2 ^ 2"`