DOCS: Fix vtk-osmesa install step (#445)

Author: SMoraisAnsys

doc/Makefile

@@ -23,7 +23,7 @@ help:
 		@echo "Removing vtk to avoid conflicts with vtk-osmesa needed for CI/CD"; \
 		pip uninstall --yes vtk; \
 		@echo "Installing vtk-osmesa"; \
-		pip install --extra-index-url https://wheels.vtk.org vtk-osmesa==9.3.1; \
+		pip install --index-url https://wheels.vtk.org vtk-osmesa==9.3.1; \
 	fi
 	@if [ "${ON_CI}" = "True" ] && [ "$$is_pypandoc_binary_installed" != "yes" ]; then \
 		@echo "Removing pypandoc to avoid conflicts with pypandoc-binary needed for CI/CD"; \

doc/make.bat

@@ -25,7 +25,7 @@ if NOT "%is_vtk_osmesa_installed%" == "vtk-osmesa" if "%ON_CI%" == "true" (
 	@ECHO ON
 	echo "Installing vtk-osmesa"
 	@ECHO OFF
-	pip install --extra-index-url https://wheels.vtk.org vtk-osmesa==9.3.1)
+	pip install --index-url https://wheels.vtk.org vtk-osmesa==9.3.1)
 for /f %%i in ('pip freeze ^| findstr /c:"pypandoc_binary"') do set is_pypandoc_binary_installed=%%i
 if NOT "%is_pypandoc_binary_installed%" == "pypandoc_binary" if "%ON_CI%" == "true" (
 	@ECHO ON