Clarify "Beta Version"/"Internal Version" flags

The value of BBL_INTERNAL_TESTING is reversed and is set to 1 when a
beta version is requested.

This behaviour got introduced in 7948040 ("ENH: update version to 1.3.0")

version.inc

@@ -7,6 +7,8 @@ set(SLIC3R_APP_KEY "BambuStudio")
 if(NOT DEFINED BBL_RELEASE_TO_PUBLIC)
 set(BBL_RELEASE_TO_PUBLIC "0")
 endif()
+# Note that defining BBL_INTERNAL_TESTING will create a beta version
+# not an "internal testing" version (the default)
 if(NOT DEFINED BBL_INTERNAL_TESTING)
 set(BBL_INTERNAL_TESTING "1")
 endif()