doc/build/gcc.rst: add missing apt-get *install*

The install command is missing for an apt-get command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/build/gcc.rst b/doc/build/gcc.rst
index fcb0b1f..1ceba0b 100644
--- a/doc/build/gcc.rst
+++ b/doc/build/gcc.rst
@@ -17,7 +17,7 @@
 
 .. code-block:: bash
 
-    sudo apt-get gcc gcc-aarch64-linux-gnu
+    sudo apt-get install gcc gcc-aarch64-linux-gnu
 
 Depending on the build targets further packages maybe needed