[format.range.fmtstr] Add ranges namespace qualifier (#5788)

The range concept is named outside of the `ranges` namespace.

Author: hewillk

source/utilities.tex

@@ -16244,7 +16244,7 @@
 \begin{itemdescr}
 \pnum
 The type of \tcode{r} is \tcode{const R\&}
-if \tcode{\libconcept{input_range}<const R>} is \tcode{true} and
+if \tcode{ranges::\libconcept{input_range}<const R>} is \tcode{true} and
 \tcode{R\&} otherwise.
 
 \pnum