[ot] docs/opentitan: index.md: add missing pixman flag to index.md

(cherry picked from commit 22e29e3)

Signed-off-by: Emmanuel Blot <eblot@rivosinc.com>

Author: pqcfox

docs/opentitan/index.md

@@ -13,15 +13,16 @@ Note: never tested on Windows hosts.
 mkdir build
 cd build
 ../configure --target-list=riscv32-softmmu --without-default-features --enable-tcg \
-    --enable-tools --enable-trace-backends=log [--enable-gtk | --enable-cocoa]
+    --enable-tools --enable-trace-backends=log \
+    [--enable-gtk --enable-pixman | --enable-cocoa]
 ninja
 ninja qemu-img
 ````
 
-* `--enable-gtk` and `--enable-cocoa` are only useful when using a graphical display, such as the
-  IbexDemo platform. It is mosly useless with the OpenTitan platform.
+* `--enable-gtk --enable-pixman` and `--enable-cocoa` are only useful when using a graphical
+  display, such as the IbexDemo platform. It is mosly useless with the OpenTitan platform.
 
-    * `--enable-gtk` should be used on Linux hosts
+    * `--enable-gtk --enable-pixman` should be used on Linux hosts
     * `--enable-cocoa` should be used on macOS hosts
 
 * `--extra-cflags=-Wno-deprecated-declarations` and