[hist] Fix more code examples with RRegularAxis

More examples I missed in commit 943ee82 ("Accept regular axis
interval as std::pair") and follow-up commit f76ad48.

Author: hahnjo

hist/histv7/inc/ROOT/RBinIndexRange.hxx

@@ -28,7 +28,7 @@ The interface allows convenient iteration over RBinIndex. If included, RBinIndex
 normal bins and RBinIndex::Overflow() is the last value.
 
 \code
-ROOT::Experimental::RRegularAxis axis(10, 0, 1);
+ROOT::Experimental::RRegularAxis axis(10, {5, 15});
 for (auto index : axis.GetNormalRange(2, 5)) {
    // Will iterate over [2, 3, 4]
 }

hist/histv7/inc/ROOT/RHist.hxx

@@ -51,7 +51,7 @@ An object can have arbitrary dimensionality determined at run-time. The axis con
 RAxisVariant:
 \code
 std::vector<ROOT::Experimental::RAxisVariant> axes;
-axes.push_back(ROOT::Experimental::RRegularAxis(10, 5, 15));
+axes.push_back(ROOT::Experimental::RRegularAxis(10, {5, 15}));
 axes.push_back(ROOT::Experimental::RVariableBinAxis({1, 10, 100, 1000}));
 ROOT::Experimental::RHist<int> hist(axes);
 // hist.GetNDimensions() will return 2

hist/histv7/inc/ROOT/RHistEngine.hxx

@@ -57,7 +57,7 @@ An object can have arbitrary dimensionality determined at run-time. The axis con
 RAxisVariant:
 \code
 std::vector<ROOT::Experimental::RAxisVariant> axes;
-axes.push_back(ROOT::Experimental::RRegularAxis(10, 5, 15));
+axes.push_back(ROOT::Experimental::RRegularAxis(10, {5, 15}));
 axes.push_back(ROOT::Experimental::RVariableBinAxis({1, 10, 100, 1000}));
 ROOT::Experimental::RHistEngine<int> hist(axes);
 // hist.GetNDimensions() will return 2