DOC: use double quotes for string parameter examples

Modules/Applications/AppMathParser/app/otbBandMath.cxx

@@ -127,7 +127,7 @@ private:
       "apTvUtBandMathOutput.tif"
     );
     SetDocExampleParameterValue(
-      "exp", "'cos( im1b1 ) > cos( im2b1 ) ? im3b1 : im3b2'"
+      "exp", "\"cos( im1b1 ) > cos( im2b1 ) ? im3b1 : im3b2\""
     );
 
     SetOfficialDocLink();

Modules/Applications/AppMathParserX/app/otbBandMathX.cxx

@@ -238,7 +238,7 @@ private:
     SetDocExampleParameterValue( "out", "apTvUtBandMathOutput.tif");
     SetDocExampleParameterValue(
       "exp",
-      "'cos( im1b1 ) + im2b1 * im3b1 - im3b2 + ndvi( im3b3, im3b4 )'"
+      "\"cos( im1b1 ) + im2b1 * im3b1 - im3b2 + ndvi( im3b3, im3b4 )\""
     );
 
     SetOfficialDocLink();