Fix code typos in documentation
Fix code typos in documentation

* documentation/quick.texi: Fix error message in two code snippets in the
      sections "Reading a Variable by Name" and
      "Iterating Over Variables in a MAT File"

CBL-ORION/matio-sf-mirror,tbeu/matio,tbeu/matio,martinichka/matio-coder,CBL-ORION/matio,martinichka/matio-coder,CBL-ORION/matio,tbeu/matio,NJannasch/matio,CBL-ORION/matio-sf-mirror,MaartenBent/matio,NJannasch/matio,telehan/libmatio,MaartenBent/matio,telehan/libmatio,MaartenBent/matio,MaartenBent/matio,tbeu/matio,martinichka/matio-coder