[mdspan.sub] Fix S::extend_type typo (#9157)

Author: abhinavagarwal07

source/containers.tex

@@ -25706,7 +25706,7 @@
     \tcode{0} otherwise,
   \item
     $e$ is the value of \tcode{S::extent_type::value}
-    if \tcode{S::extend_type} is a specialization of \tcode{constant_wrapper} and
+    if \tcode{S::extent_type} is a specialization of \tcode{constant_wrapper} and
     \tcode{0} otherwise,
   \item
     $t$ is the value of \tcode{S::stride_type::value}